I have no idea how to romance my best friend; she always say that she's not into me. I also don't know how to romance the teachers.
Best Friend romance is tied to their events, you can find out more on the wiki. Not sure how to romance teachers, all I know is how to raise lust and possibly have sex with them.
You could romance the teachers also before the last contents..
You can meet your teachers and also the others in River Rat or the city..
When in River Rat if they sitting at bar you can offer them a date, propose to them as any other NPCs..
Missing just milestones for them..
About the BBF..
1) Talk about sex
Then will open 2 paths..
a) Popularity at Quad Parties then solving the Milkshake Mistery
b) After 2 weeks while showering during the weekdays (7-8.30) there will be a chance all your clothes will be stolen..
BFF will help you, the will need to wait an RNG where will ask help with an essay.
You can still choose to solve the Mistery after the event in shower..
This will unlock relationships with BBF..
Out of thay you can hangout/date the BFF in any moment, but sex will be unlocked only after you opened the relationship option in any manner..