The best thing to do is to put only one class in a database file.
Here are the instructions to do this:

1) Start up your gradebook and use the FILE - SAVE AS option to
make copies of your database.  Call each copy by the name of one of
your classes (but do not use spaces and limit the name to 8 characters).
2) Now use the File - Open command and open each of the files you
just created in step 1 and delete all the classes in the file except
the one which corresponds to it's file name.
3) That's it.  Now when you want to switch classes you will need to 
use the FILE - OPEN command.

Now test and see if your performance is improved enough to warrant this
more elaborate setup.  You still have your original file so you can revert
back to your old setup if desired.