There are reasons to disallow USB/external/removable drives. A lot of malware enters corporate networks through these devices. Someone gets an infection on their home PC, connects their thumb drive to it, and then brings in the drive to transfer data to their work machine. In a school environment you're especially in danger from the student population-- both in terms of unknown home security practices and potential malicious intent (key loggers, etc.).
Also, allowing these devices is another way that corporate data can leak. Not to say it can't be bypassed or that other ways of getting the data out exist, just that it's one way.
Companies have to safeguard their data or risk becoming the next ChoicePoint. Where I work, I'd never approve (I'm the security officer) a user bringing in their home PC. Any machine allowed on our network has to meet our minimum security baseline.