Right when class ends. Go up to them, sit down or just tell them to sit down and introduce yourself. Be like "hey im [l][o][s][e][r], can I have your number" and blah blah, and then cool go to your next class.
Just simply be confident and ask them. If they say sure, you can now text them without having others listen to you guys talk if youre nervous about that. Then through text get to know them. You'll probably end up texting him through some of your classes and then at the end of the day just say "ill cya tomorrow" or whenever you guys have class together.
Just slowly keep that up and eventually you'll be sitting next to eachother, then slowly you'll walk to classes, then you'll hang out after school boom you have a boyfriend
like im not even lying its really easy, literally just go up to them after class and politely ask can i get your number. and text them during the same day.
(if its your last period with him) same thing, just text him during the day after school).
dont say you like him or anything tho