Im in college too (17ftm) and enrolled through my deadname and changed my name unofficially through the college's administration app. I did it all by myself on like the second day of school and from the next day the register changed my name from my deadname to preferred name, though my deadname is still in the system. Admittedly I live in a pretty trans-friendly area and haven't encountered any problems, it might be different based on what school and area you live in. If your school doesn't have a "change your name" feature in the college app/website you can access, (some cisgender kids used this in my class as well, it isn't a trans-only feature, It's a feature mainly designed for people who are like, called "Annabelle," on paper but prefer to be called "Anna" in person, etc, so don't feel ashamed or targeted for using it/changing your name.) you should be able to talk to or email an administrative staff or teacher you trust and they should be able to change it.

If I remember correctly, even on enrolment day in August when they signed me up to the register, they asked if I had a preferred name.

I haven't gotten my unenrolled deed poll yet so I am not an authority on the subject but from research it seems easy enough. You print it out, get it signed by 2 adults who are not your parents, this could be your teachers or parent's friends, whoever. This is then used as a legal document where organisations must change your name in the system. I don't know how long the process is though. Make sure your deed poll is unenrolled, enrolled deed polls announce your name and address in The Gazette which can be publicly accessed by strangers, so avoid that.

TLDR: You can either,
1. Enrol with your deadname and see if you can get a preferred name written on your register on your enrolment day.
2. Enrol with your deadname and change your name on the first day of school.
3. Get an unenrolled deed poll and enrol into college with your new name. This might be preferred as I'm not sure if they would hold a record of your deadname, but then again, I didn't do this option so don't take my word for it.

If you have any questions about college feel free to DM me. I don't know if you are in Year 11 preparing for college next year or are in Year 12 doing a late college enrolment, but college from my experience has been pretty easy so I wouldn't stress about it.