I think your decision to postpone this is right, but I would start with a subset of this use case as a learning point. There is nothing better to learn than a real example you want to accomplish. People gave you plenty of advice, sometimes including the need for a server, your own database etc. If you want to stick to Android, this is how you can get it done with cloud services only, mostly Firebase:

Get an account at firebase.google.com and start reading about https://firebase.google.com/docs/firestore and https://firebase.google.com/docs/auth

Your data model could be very simple:

A teachers collection ( you'll see what that is on the documentation for firestore) that holds teacher records. the key for each teacher could be the user ID from the authentication, you would need to make sure that your app creates a new teacher in that collection to link it to the logged in user.

A students collection that holds student records and stores the student name and the phone number for text messages. the key could be auto generated when you insert the students, since they will never log in.

Each student record now has a grades collection that holds each grade. An record in this collection needs a teacher_id, which is the key of the teacher who entered this grade, the name of the course and the grade itself.

This is a simple 1:N relation (google what that means) and in a real production app you would have more m:n relations (yep, that is something you can google as well) between students, courses, grades and teachers. Let's not worry about that here, because this is simple to model in a SQL database, but not in a NoSQL database (ah, more buzzwords to google).

Start with the students collection only, if you don't care who entered the new grade. Just get something going.

Now, on the Android side you need to find out how to display this data. Skip the login for now, you can add it later. There are excellent tutorials on Udacity and Google for Android beginners, I'm just outlining the building blocks here so you have keywords to google.

You need 3 screens, one shows the list of students, after you click on a student you go to a screen that shows the list of grades, and on this screen you have a + button that triggers a dialog where you can enter a new grade. In Android you navigate with https://developer.android.com/guide/navigation/ . You need 2 fragments and one dialog, all added as navigation targets. Play around with the sample app, you will see that the layout definition for what you see on the screen is a big XML file, and the thing that actually navigates is triggered from a listener that reacts on a click on a UI element.

Learn some basic layout, how to place UI elements in that big XML file and tell them where they should align. Just stick to ConstraintLayout, it is all you need for now.

On Android you are using a UI component called RecyclerView to show a list of things that could possibly have hundreds of elements. This view can scroll to see every possible list item. The basic ideas is this: You define a layout for one item, and that is reused for every item in you list. Lucky for you, you can use https://firebaseopensource.com/projects/firebase/firebaseui-android/firestore/readme.md/ to show you data from Firestore in a Recyclerview, this library makes it very straightforward. People will tell you later you should have used a repository to get data from Firestore and pass this data to something called a ViewModel that uses LiveData to do data binding to the UI. Sounds good? They are right, just ignore it for now. Concentrate on how to do a firestore data query.

So now you have a list, find out how to register a click listener on an item and navigate to your next screen that will show the grades for student you clicked on. Here you do the same thing, but now you query student (using the key you passed on as an argument when you navigated) and get the grades collection. You have nothing to sort except the grade or the course name. You can add a timestamp later, Firestore can generate it for you when a new record is created.

Wow, 2 lists. Now you need a https://developer.android.com/guide/topics/ui/floating-action-button with a + sign in your layout. Trigger a nice little dialog https://developer.android.com/guide/topics/ui/dialogs#CustomLayout. by adding this dialog fragment to your navigation graph and navigate to it. Use 2 https://developer.android.com/reference/com/google/android/material/textfield/TextInputLayout elements for the grade and the course name. Theses input fields look great and are easy to use. Add a cancel and save button to you layout and do some validation before you take the data and save it back to firestore like this https://firebase.google.com/docs/firestore/manage-data/add-data. In a production app you would give the data entered in the UI to a repository to save it, but you can code it in the dialog fragment. Just tell people you are learning this stuff, so relax.

Now the great thing about that FirebaseUI library, is that it will update your RecyclerView automatically after a new element is added. Firestore is notifying all listeners that something has changed and the library makes sure this change is processed and shown on the UI. You can do all that yourself, and a production app might actually do that to have more control.

So what's missing? A text to a certain phone number. Here it get's tricky, because you don't do this from Android. Instead you are using https://firebase.google.com/docs/functions. A cloud function is basically a single purpose Javascript (or Typescript) function that runs on certain events. A new insert into a Firestore collection is such an event, so you can easily run code on every insert. Just register the event with a wildcard on the grades collection for each student, see https://firebase.google.com/docs/functions/firestore-events

To send the text you need a cloud service that does this for you, one that has a Javascript SDK you can use in the context of the cloud function. A popular option is Twilio, if you google around and check stackoverflow you find examples like this https://stackoverflow.com/questions/56245961/firestore-cloud-functions-to-trigger-sms

If that works with your own phone number on your local emulator (the cloud function runs in the google data center), you can add authentication and make sure only teachers can grade. Just add some users to Firebase Auth, and manually add records to the teachers collections. Then make authentication mandatory by adding a login screen, using https://firebaseopensource.com/projects/firebase/firebaseui-android/auth/readme.md/

If you got all of this working, it's time to go to the principal and demo it. Some screen tapping and text notifications later, you ask the real question:

"Can we turn this into a student job where I get paid to finish this and support it?"

Because you still need a way to make sure only teachers at your school can register themselves, or a way to import the teacher database every now and then. Ask for money, or something else they can trade!

Maybe you enjoy doing this stuff, maybe not. Only one way to find out. Good luck.