So, you want your bookings table to look something like:
id, teacher_id, time
1, 1, 18.00
2, 1, 18.05
3, 1, 18.10
4, 1, 18.15
Is that right? If so do you need to do it on the fly, ie a time is submitted, and then 5minute slots are allocated from there, or it will always be between 18.00 and 20.00.
If it's the latter, I'd make up a table called slots (id, slot) values (1, 18.00), (2, 18.05), (3, 18.10) etc... then have a look-up table teacherslots(teacherid, slotid), and link teachers to time slots that way.
If it's on the fly, then I would do it in PHP and calculate the time slots and add them to an array. You can then generate the sql query using that array and submit it.