Question

1. We learned math and science.
2. We learnt math and science.
(Which past do we use? Are both okay? Which one is commonly used?)

3. I went there every other week.
4. I went there every second week.
5. I went there every two weeks.
(Are the three sentences the same?)

Answer 1

Both 1 and 2 are correct.
"learned" is standard in American English.
"learnt" is more often used in British English.

Yes, 3, 4, and 5 all mean the same thing.