There is usually a plate inside the port door on the side of the machine that will tell you which machine you have. Here are 13 different ones: http://www.everymac.com/ultimate-mac-lookup/?search_keywords=M5521
Select which one you have to see what the original operating system was and just how high it can go.
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The school may still have the original installations disks. If not, you will be able to find them on eBay.
There is usually a plate inside the port door on the side of the machine that will tell you which machine you have. Here are 13 different ones: http://www.everymac.com/ultimate-mac-lookup/?search_keywords=M5521
Select which one you have to see what the original operating system was and just how high it can go.