Implementing Trigger problem

Hi,

No matter how i read the manual i can't understand it. :(

Here is what i would like to do.

for example i have

table:   classes
column:  classno,...,schedule_count

table:  schedules
column:   classno,time

I would like to create a trigger that everytime there is an insert
or delete in schedules
classes.schedule_count will be updated with number of schedules
entered in schedules for that particular class.  this way i could
track the number of schedules for that particular class
without counting the schedules.

how do i implement it?

thank you,
oscar