My guess is that the Principal at Carroll decided that it was important to have Biology teacher so they decided to spend their money on that resource.

I know that most middle schools offer Algebra 1 for advanced students, but only a select amount of middle schools offer Biology in 8th. The ones that I know are Kenmoor, which I thought was because of it's TAG program, but apparently Charles Carroll has a Biology teacher too, and I don't think it has any special programs. Why do only some students have access to Biology teachers?