Q: Phone and Calculator gone. Same day, emailed about phone. Test next day. Day after. Received email about calculator.

A: It is unclear why you are complaining that someone is trying to return your calculator, or why you think this is a legal problem. There is no evidence in your post that your Instructor had your calculator the entire period of time it was missing, or knew that it was yours. You don't explain why you are afraid of the other students.

With regard to found property, California Penal Code 485 PC states "One who finds lost property under circumstances which give him knowledge of or means of inquiry as to the true owner, and who appropriates such property to his own use, or to the use of another person not entitled thereto, without first making reasonable and just efforts to find the owner and to restore the property to him, is guilty of theft."

There is no reason to believe that your Instructor, if he had possession of the calculator during the entire period, would not have returned it when you showed up for your Midterm. The efforts he must undertake to return found property are "reasonable". He doesn't have chase you down. Further, you could have borrowed a calculator from someone, or simply purchased a new one, to take the Midterm, rather than wasting your tuition and withdrawing entirely.

A: much more info needed.

if a teacher just found it then it does not sound like anything was done wrong.

if a teacher took it, then no that is not legal. However, your explanation is far from clear.