Ok, this is your mother waaaaaaaay overstepping her boundaries. If you don't lay down your own boundaries ASAP, this kind of behaviour will continue as you move into adulthood. I have an overbearing mother and father. My solution to dealing with them was to move halfway across the world after graduating from university, as it was literally the only thing I could think of that would get them to stop trying to micromanage and control my life.

You need to be firm with your mother on this. I read in another post that you are worrying she will escalate the argument or break down and cry. Let her. If you don't stand your ground now, she will continue to walk all over you.

You can be tactful, but if she chooses to be an immature petulant child, that is her problem, not yours. You need to be firm, and do NOT waver. If you cave at any point, she will learn that you are easily manipulated and seriously keep overstepping. I have read stories of overbearing mothers and mothers-in-law (check out r/justnomil or other related subreddits for what your life could be like if you don't put a stop to this now).

This is seriously as simple as saying: Mum, I understand your point of view, and I understand that as a mother, you worry about me. But, I am an adult, and I am making an adult decision right now. I will be walking to school. That's the end of this conversation.

If she doesn't let up, just ignore. If she shows up at your door to drive you to school, don't get in her car. You need to be serious about this, or she will continue to unleash her crazy on you. You don't need this stress as a student or in the rest of your life.