There are a number of options you can use to clear your disk, with varying degrees of success.
The method mentioned above is one that's pretty common - zero fill or overwrite your entire harddrive with either 0s and 1s using 1, 2, 7, or 35 passes. However, there are a number of articles out that claim even 35 passes won't ensure that your data is fully secure (especially in bad sectors of the hdd which would've been sectioned off and where the formating couldn't easily access).
The recommended method is to physically shred the HDD. There are some industrial tools for doing this, with a combination of strong magnets and metal shredders. Alternatively you could simply destroy the HDD using a hammer and nails. Be sure you destroy the actual disks inside the casing and not just the head and outer enclosure (i.e. drive many many nails through the thing). Soaking it in water and other chemicals helps, but physical damage is the best bet.
This all depends on just how far your student would go to access your old data of course. If you use the 7 or 35 pass method above, it's doubtful he could recover all your files in their entirety, but maybe just fragments. Of course, if he hires a professional data recovery company or gets the government involved, the chances of recovering your data can be substantially increased - this applies to everything short of fulling shredding the disk. Good data recovery companies can recover even waterlogged HDDs.
Obviously, physically shredding the HDD and replacing it with a new one would be the safest method.