FDISK

I teach PC repair and have been frustrated with changing OSs for students. I am familiar with fdisk, PM and just plain old del tree, but it appears that when I go to setup the new OS detects the presence of NTFS or Linux ect or will not delete logical drives in the extended DOS partician. It has taken several deletions of the partician and reformating several time to get a OS setup to run. Is there a utility out there that can wipe a hard drive clean from a boot disk?