Can I ask my theatre teacher for one of my main rec letters or should those come from academic classes?

Answered

Even if your theatre instructor knows you better than say your Economics or History or Math or Science teacher, it might not be the right move if you are planning to apply as a business major. If you were applying as a Theatre or Performing arts major, your Theatre teacher would be best.

Business schools are very competitive like engineering schools, especially at the undergraduate level. At Wharton, I think the admit rate is less than 4% versus the overall UPenn admit rate of 5.7%, and that makes sense because Wharton carries a lot of "clout". If you are applying to a business school within a university system, then you want some who can advocate for you as a savvy, astute, clever, entrepreneurial leader who is suited to succeed in business. If your other ECs or Spike(s) are aligned with a business school that would greatly help as well. The reason I say this is that at Wharton for example, they have application readers from Wharton reading your file to see if you are a good fit for Wharton. So you want the recommenders to appeal to the audience reading the applications.

If you are applying to liberal arts college where you don't have to choose a major until your 2nd year, the nature of the recommenders matters less because they are looking for the best students regardless of major.

Good Luck