Ummm 学校 actually isn't always used for schools. And 大学校 really isn't that common. You're much more likely to hear 大学（だいがく） . A better example might have been 高校（こうこう）which translates as high school.
Basically, both kanji (学 and 校) are used depending on what type of school you're talking about. You'll find that there are alot of kanji that have a specific meaning but aren't used by themselves. Kanji are alot like english prefixes and suffixes, in that they usually function as complete ideas on their own but often can't be used without something to attach to. This isn't always the case as 子 and 友 are words in their own right. However, you couldn't use 校 by itself.
I guess ultimately it is a pleonasm, but not really because 校 doesn't mean school unless 学 comes before it. I hope that makes some sense.
Ummm 学校 actually isn't always
Ummm 学校 actually isn't always used for schools. And 大学校 really isn't that common. You're much more likely to hear 大学（だいがく） . A better example might have been 高校（こうこう）which translates as high school.
Basically, both kanji (学 and 校) are used depending on what type of school you're talking about. You'll find that there are alot of kanji that have a specific meaning but aren't used by themselves. Kanji are alot like english prefixes and suffixes, in that they usually function as complete ideas on their own but often can't be used without something to attach to. This isn't always the case as 子 and 友 are words in their own right. However, you couldn't use 校 by itself.
I guess ultimately it is a pleonasm, but not really because 校 doesn't mean school unless 学 comes before it. I hope that makes some sense.