Question

1. We have three PE classes a week.
2. We have three P.E. classes a week.
3. We have three pe classes a week.
4. We have three p.e. classes a week.
5. We have three Physical Education classes a week.
6. We have three physical education classes a week.
[Can we use all the expressions? Which ones are unacceptable?]

1.We have three PE classes a week.
8. Thee are three PE classes a week at/in school.
[Does #1 mean #8? Can we use both 'at' and 'in'? Are they the same in meaning?]

Answer 1

If Physical Education is the formal name of the class, then 1, 2, and 5 are correct. However, if you're referring to physical education in general, and the actual classes have distinct names (Girls Physical Education 9, for example, or Boys Soccer 11), then 1, 2, and 6 are correct.

If you mean "There" at the beginning of 8, then yes, 1 and 8 have the same meaning. Either "at" or "in" are fine.