Advanced Placement (with the test) or College High (offers college credit, often via local community college) classes are always a good idea. I took College High Biology, wound up with 3 college credits from Lane Community College, and managed to opt out of my college math/lab science requirement, which was AWESOME. I wish I'd taken the AP English test (took the class) so I could've done the same with my English Comp requirement.
In high school your teacher will be much more accessible, and help easier to find. College courses are larger, faster, and exponentially more difficult. Take it now, while it's free, while you know your teachers, while you can gain access to teachers for additional help/tutoring, and while you can maybe eke some free credits out of it. If you have to take it again in college, at least you've done it once and have a decent foundation. If you can test out, or advance more quickly, so much the better.
Isn't vet school a graduate degree (Masters) program? Specializing in biology and chemistry at the undergrad level (Bachelors Degree, 1st 4 years) is likely wise.