I was charged with trespassing on school property (1st offense). My question is: the police did not actually catch me in the act of trespassing - a witness called to report seeing

Check out the Arizona statute for Criminal Trespass. It doesn't mention who catches you if you are trespassing.

A.R.S. § 13-1504. Criminal trespass in the first degree; classification

A. A person commits criminal trespass in the first degree by knowingly:

1. Entering or remaining unlawfully in or on a residential structure.

2. Entering or remaining unlawfully in a fenced residential yard.

3. Entering any residential yard and, without lawful authority, looking into the residential structure thereon in reckless disregard of infringing on the inhabitant's right of privacy.

4. Entering unlawfully on real property that is subject to a valid mineral claim or lease with the intent to hold, work, take or explore for minerals on the claim or lease.

5. Entering or remaining unlawfully on the property of another and burning, defacing, mutilating or otherwise desecrating a religious symbol or other religious property of another without the express permission of the owner of the property.

6. Entering or remaining unlawfully in or on a critical public service facility.

B. Criminal trespass in the first degree under subsection A, paragraph 1, 5 or 6 is a class 6 felony. Criminal trespass in the first degree under subsection A, paragraph 2, 3 or 4 is a class 1 misdemeanor.