If the hard drives are platters, meaning they spin and are mechanical, use DBAN. If the hard drives are SSDs then you need to purchase Parted Magic. Parted Magic is a bootable CD. You would burn Parted Magic to CD with something like Imgburn portable and then boot the CD like you would a Windows install. Then in Parted Magic find the option for a secure erase. The reason why you need to use Parted Magic for a SSD is because erasure programs like DBAN make hundreds if not thousands of writes to the disk. With SSDs that would wear out the drive exponentially. So you do NOT want to run something like DBAN on a SSD. Also note, DBAN is also a live CD and you boot it as well.
Keep in mind that once you erase the drives you'll have to reinstall the OS. Unless of course you just give the computers to the school with no OS and let their IT department install Windows themselves.
Edit-
Depending on how large the HDDs are, it could be an all night affair just erasing the drives. So keep that in mind. Small price to pay to keep your Info. from being recovered.