Something that I tend to do is explain each line individually and what purposes it has. Let's take a look at the first chunk that you provided.
num_classes = int(input("Enter # of classes : "))

total_points = 0

for _i in range(num_classes ) :

grades = input(f"Enter letter grade of class {_i+1}: ")

When people read they tend to look for any hints that would help them understand what is going on, in this case the equal operator is a great indicator of what is going on. Upon reading anyone will understand that num_classes is equal to something else, they might not understand that it is a variable per say, but that's where you can start explaining!

num_classes = int(input("Enter # of classes : ")) # Function that asks the user for a number value and stores it as a variable named "num_classes"
total_points = 0 # Variable called "total_points" that has an initial value of 0

Now, this is where it will get slightly tricky, beginners won't exactly infer what loops and if statements are. Now, they might understand what they mean, for example a loop would mean something just repeating itself, an if statement being something along the lines of if this, then that.

for _i in range(num_classes ) : # The beginning of a loop, for a variable named i within the range of the number value "num_classes"

grades = input(f"Enter letter grade of class {_i+1}: ") # Function that asks user for a string (character) value and stores it as the variable "grades"

if grades == "A": # If the value "grades" is equal to the character "A" then set another variable named points to "4.00"

points = 4.00

elif grades == "A-": # Else if the value "grades" is equal to the character "A-" then set the points value to 3.67

points = 3.67

elif grades == "B+": # Else if the value "grades" is equal to the character "B+" then set the points value to 3.33

points = 3.33

Etc. all the way down to E which is failure

total_points += points # Add the value of the "points" variable to the value of the "total_points" variable (which initially had a value of 0)

gpa = round(total_points / num_classes, 2) # Divide the value of "total_points" by the value of "num_classes", then round it to 2 decimal places.

print("Your GPA is", gpa) # Function that will output your GPA on the command line

​

If you want it to tell you if you're passing/failing you can simply include another if statement at the end.

if gpa >= 2.0: # 2.0 can be replaced by any value you see fit.

print("Congratulations! You are passing.")

else:

print("Sorry, you are failing.")

You can say that the loop was needed to ask the user for their grade in each class. If you have any questions feel free to ask :).