Controlled access for a place that 200 to 2,000 people need access to everyday may be relatively simple, but it's not necessarily easy, especially if you are talking about not letting in crowds.
And of course it's worse with schools that have "campus" designs rather than a school building. Our local elementary schools are pretty easy to have security for. Our high school is a security nightmare. Even if you fenced the whole campus with barbwire on top, there are so many different places to go on campus. Trying to restrict entry by car to one or more entrances that have security checks would be a logistical nightmare. And then at the end of the day, I suspect the biggest risk is still from existing students, so if you're not searching cars and people on teh way in, you need multiple security screening points around campus.
I think with that type of set up realistically all you can do is have a badge system and armed security so that you hopefully will notice any one that doesn't belong more quickly, but I don't think you're going to be able to stop them from entering campus and getting to a building if they want.