How to make acquaintances friends?

Okay so I'm 15 and have many acquaintances they are just people I talk to at school. I hang out with them at lunch and partners and things in cl but we don't hang out outside of school I don't know what to do to hang out. Like today was the first day of school and I just made 2 more people in homeroom and sat with them at lunch. I probably have 10-15 acquaintances and I'll probably meet more people this year. So I don't really know how to make friends with these people I talk to at school