A ratio of 20:1 would mean 125 teachers and a ratio of 100:1 would mean 25 teachers. Perhaps around 100 would be reasonable?
I just realized a disconnect... those ratios are per class. That is assuming that all the students take a full schedule of classes in a day.
2500 students is more than I had guessed. I wonder if there would be room for more than one such institution?
2,500 is a rough guess, assuming a 6-year window (18ish to 24ish) for most of your students, and a demographic curve that is high in youngsters. (A near asymptotic curve,
like this one, with increasing age along the bottom axis.)
You didn't specify the source of your population growth, so I assumed that one so dramatic had to have a lot of immigration. And, I assumed that immigration would look a lot like frontier immigration in US history: youngish families (or pre-family) and lots of young adults. Some percentage of those might end up in your university, as well as a goodly chunk of the youth born there.
(Y'all feel free to beat up on my numbers, there. I haven't crunched anything hard - just some TLAR [That Looks About Right].)
If you want your school to be more elite (not teach AgriBusiness and Mining and Materials Engineering, but Medicine and Philosophy and Physics) then it will have a smaller student body.
If you are assuming a really small college, then you might have more than one. Or, you could have one university with multiple campuses and multiple colleges.* Your administrative staff demands go up a little when you do that. If you go with the larger student body, then you might have a campus/college in one area that teaches AgriBusiness, and another that teaches Mining.
I think graduate degrees in that small of a population would almost have to be off-world. (It appears 11% of Americans in 2011 had a Masters or Doctorate, and some percentage of those are in Puppetry or Ethnic Studies or Navel-Gazing or some such - which are simply not going to happen on a colony that you describe.) The infrastructure to support those is going to be too large for the small quantity you get. You can either import all your doctors, or you can send them off somewhere to return once they have their degree.
* I am using the old-style concept of University being the institution, and Colleges being divisions within the University. There are other uses distinguishing Universities from Colleges, too. (I refuse to ever take a class from UMUC - any school that calls itself University of Maryland University College just can't be trusted to know what the heck it's doing!)