Whatever you do, do it gradually. You do not want ot look like / be accused of exaggerating without reason.
The exact steps are subject to change, according to your current daily situation.
NOTE: you may go the "most extreme" solution, if your colleague's behavior becomes excessively aggressive (psychologically or physically).
Keep telling him (in private, if possible), whenever needed / appropriate, that you are not interested in him (romantically) and that he should stop. If private communication is not possible, go to step 2. NOTE: you should not try to contact him in private - do it only if it happens -> if he comes to you with his proposals.
Repeat step 1, with other colleagues around.
Involve a supervisor from your school. ( I do not know the organization of the school, so I cannot tell you the name of the right person. Ask some teacher / professor about who is the most recommended person to talk to about this.)
Tell your parents. Tell them what you did (each step), and which were the results each time.
Your parents will take over and they will know what to do next.
Of course, you can talk to your parents from the very beginning. They may have some good advice too - and they already know many details about your life, colleagues...
NOT talking to your parents can be useful IF the situation is not out of control. Although unpleasant, having a picture taken (across a lunch room) is definitely not life threatening. Also, parents may overestimate the situation and act with more "force" than necessary - maybe hurting a bit your image in front of your colleagues.
The BENEFIT of doing this by yourself is that you will learn to deal with things by yourself. You will learn to control your emotions while talking, and to make a good point when talking while under the burden of negative emotions.
Last, but not least, you can just ignore him. As long as he does not become violent, he will eventually lose interest.
AGAIN: if the situation becomes dangerous in any way, talk to your parents and to you supervisors at the school. No image and no other thing is more important than your safety.