Will iPhone alarm sound if you're on a call or FaceTime?

Not sure if I’ll go off & scared to fall asleep on the phone because I have school tomorrow & need to be on time for the bus

Yes, ur alarm will go off and it won't wake the other person

You fall asleep while on the phone and the other party doesn't hang up during all that time?

I've seen posts here with people complaining about an alarm they have set going off while on the phone or while doing something else. I'm pretty sure an alarm that is set will sound off regardless which is by design so the alarm isn't missed.

Yes, actually The alarm clock will simply pop up and wake you up. Just be sure to have the ringer on and if you feel skeptical try first with a one minute timer while on facetime. If the timer goes off then it will work for the alarm as well. If it doesn't then it doesn't work

I went to sleep on FaceTime and my alarm did go off on time. You could always see for yourself. I have 6s

I don't know for sure -- but it would be very easy for you to test. Just set an alarm for a few minutes from now and duplicate the situation you are wondering about.