A couple of thoughts:

The cutting up of input stream as you describe can be much done much easier with sed, for instance, instead of

query-students | sort | unhead '^D' | untail '^G'
[download]

do

query-students | sort | sed '/^D/,/^G/p; d'
[download]

Also, neither head nor tail modify files they operate on. I understand, you were probably solving a specific problem, but this script can be generalized even more.