Gertrude - I dont think its that simple to be honest.
Student and Instructor A have a plane booked for a 2 hour slot and they plan to land away. After a brief, walkaround, startup etc, they take off having used 30 minutes of their slot already. They fly for half an hour, land, get out and pay, startup again and get airborne again, after landing they de-brief they hand the plane over to you 2hr 20 minutes later. Thats 20 minutes of your slot gone. You now have 1 hr 40 minutes.
You also planned a landaway so you are now under pressure to get going, land, turnaround quickly and come back. It was supposed to be a fun relaxed flight with friends and now it isnt. However, you didnt rush because you wanted to be safe so you too land 20 minutes into the next persons slot.
Student B and Instructor B are now unhappy with you. Do they really care that it was the person before you that caused the delay? Probably not.
Take a nice sunny day, lots of people are using the planes and delays inevitably creep in and invariably increase as the day goes on.
Thats the frustration, and in my opinion you either book 3 hour slots to try and counter those delays, or try and get into a share. Changing the system and having everyone being on time or not over using their slots isnt easy.