Teacher (globalroles mrbsbooker) can book a free 1. place two periods the second is allready booked.
the second is marked in read color unconfirmed

I will not that teacher can book an already booked period.
The globalrole does not accepted forced booking only the following are allowed.
block/mrbs:editmrbs allowed
block/mrbs:viewmrbs allowed

in moodle 2.3 teacher didn't tell me any issue

should I first install moodle 2.4.1