@Bullet700 Relationships are complex. They don't just instantly happen. A GOOD relationship needs time for trust,love,and care to form.
I would try to be friends with a girl/girls first. Be a good caring friend and build up trust with them. She may friendzone you but it is one step closer.
Sometimes girls find you attractive but they want to play hard to get. This is very common. You can be persistent but not too persistent or they find you creepy.
Like you said you have been in a all boys school. You may not understand how girl's emotions work. Girl's emotions are delicate and can suddenly change.
Just try to build up good friendships at first then go from there. Good Luck.