I'm not sure if I follow you. Is someone else walking inside your apartment? And you are pretty sure you have locked the door? Perhaps the lock is broken and it really is popping open? It happens to my door once in a while if I don't use the deadbolt.

You live in a house w/ 2 apartments? Perhaps a main apt. (yours) and a sub-apartment (for the tenant)?

It sounds like either she has keys (in which case you need to tell your mom or whoever the person is in charge of the space) and get the locks changed, or she's telling the truth, the door keeps popping open, in which case the perso in charge of the space needs to call a locksmith to repair it.

it really doesn't matter if she's been coming in and out--you (your mom, maybe) need to change or fix the lock.

I wouldn't bother with setting a trap--I'd just get the situation changed so no one can come in.

Change the keying for that lock, or put one of those "chain locks" on your side.

For now, just slide a heavy piece of furniture over the doorway.

Here is a link that might be useful: door chain lock

Don't know what grade in school you are in, but I think you need to make every effort to attend class in the future. I read your post twice and am still not sure just what happened.
Linda C

get some big ol' jingle bells and tie them to the door knob...so you can hear them opening the door or buy a huge dog to attack!