Let's think about this -
15-20 minutes for a pre-flight brief, you do brief your student don't you?
15-20 minutes to sign out the aircraft, do a thorough walkround, pre-flight and start up, you do carry out a thorough walkround and pre-flight don't you?
60 minutes chock-chock
15-20 minutes for a thorough after-flight, walk in, sign in and debrief, you do debrief don't you?
Total 1.45 - 2.00
Do that 6 times and you have a 10.5 to 12 hour day - how fresh and effective is your instruction going to be on the last flight?