Inform or Obstruct?

Yesterday (April 1st) at my high school two girls pulled a fire alar, causing the school to be evacuated for about 45 minutes in the middle of the day. The girls were apprehended.

During the next class, the alarm went off again, but this time the school was only evacuated for about 5 minutes. The Principal and Assistant Principal, heading the "investigation" do not yet know who is responsible.

After that second evacuation, a good friend of mine who I would rather not lose told me who did it. He sounded sure and had just been talking to the person. He told me to tell no one.

Today, (April 2nd) a semi-friend of mine told me he was being accused of that second alarm-pulling. He's been in trouble before and the police officer who questioned him said he would tell his probation officer about this if this semi-friend couldn't come up with something proving his innocence within a week. After he told me all this, I said I think I knew who pulled the second alarm, to back him up in a way, yet I can't tell him because my good friend has trust in me.

So I'm not sure what to do. I don't want my troubled friend to get into more trouble for something he probably didn't do, but I also don't want to lose the trust between my good friend and I.

I considered telling the Assistant Principal that I know my friend is innocent because I know the real person responsible, but I can't tell him who that person is. The problem with that is that I'm afraid the Asst. Principal may feel forced to interrogate me, or, worse, accuse me of pulling the alarm.

What do I do?

id say, talk to that good friend of yours and see if theyll go tell the asst principal or whoever whatever he knows. maybe offer to go with him, if the two of you go together and inform the faculty of your knowledge then you wont lose trust in that friend, and at the same time youll help get your other friend out of trouble.

or you could just wait it out, i doubt they will be able to do anything to that friend of yours anyway if he was in no way involved. if there is no evidence then they cant do shit... right?

That's not really how it works at my school. If you've been in trouble before, you have the stigma of being a bad kid and everyone blames for the next bad thing that happens. Thanks for the suggestions though.