I'm no expert, but my gut instinct is that if they're friends with the girl that accused you, they ARE setting you up for something. So yes, don't go anywhere alone (the more witnesses, the better), and keep some kind of record of when & where they're following you (but be nice to your friend) - it may be good to also take notes of where you go. Do not approach them for any reason and avoid physical contact at all costs. If there is anyone older or in a position of authority you trust (an older friend, teacher, etc) talk to him/her asap, say you think they're trying to set you up for something (never say you're being bullied or harassed because society tends to think females can't do that to males - say that you're concerned about being set up, not concerned about the 'surveillance' in and of itself). if you can, travel by car and not on foot - if you can't, ride a bike (to minimise chances of their bumping into you). If you decide to complain to the school authorities again, it may be a good idea to request an appointment in writing (or for your parents to ask for one if you're a minor), but it may only be worthwile if you have a 'respectable' witness (an older adult who is not your relative - but bear in mind any witness is still better than none). avoid routines - the more predictable your movements are, the easier it's for them to set you up. this particularly applies on foot. always take a phone with you.
on the bright side, once you graduate you'll probably never have to see them again.
_________________
I am the steppenwolf that never learned to dance. (Sedaka)
El hombre es una bestia famélica, envidiosa e insaciable. (Francisco Tario)
I'm male by the way (yes, I know my avatar is misleading).