If you are interested in joining a college equestrian team, what I would do is compile a list of 5-10 universities you would be interested in attending that have a good veterinary program. Then I would look at the websites of those schools and determine which ones have equestrian teams, and whether they're NCAA (a bit more competitive) or IHSA. NCAA may be a bit tougher to get into--for instance, the local college here that has an NCAA team requires auditions, and probably 90% of the team members already have extensive experience competing in rated and/or breed shows. In other words, it's not impossible, but it's definitely tougher to get on an NCAA team than an IHSA team. Another option is to look at schools with polo teams.
I know this may sound like silly adult advice, but I highly, highly recommend choosing your university based on their academic programs. Try to get into the best school you can manage with whatever your grades might be. An equestrian or polo team should certainly factor into your decision, but it should absolutely be secondary.
And if you find a school that has a perfect veterinary program but doesn't offer a riding team of some kind? Go there anyway, find a local barn, and continue taking lessons.