You can get done with classes table faster, when you use an append query.
Btw. don't use field names with spaces in them, i.e. instead "Class Number" use "CalssNumber" or "Class_Number".
Create a temporary table with a single field Class, p.e. tblTempClasses: Class, and fill it with all classes for one semester and course.
Create a saved query p.e. qClearTempClasses: DELETE FROM tblTempClasses
Create a saved query (I created the example for semester with ID=7 and course with ID=1) p.e. qAppendClasses:
INSERT INTO tblYourTableName (ClassNumber, SmsID, CrsID)
SELECT Class, 7, 1
FROM tblTempClasses
NB! Never include autonumber into INSERT query!
Run the queryqAppendClasses - the classes from temporary table are added into your table. If you created the unique composite index for fields ClassNumber, SmsID and CrsID, the query works when no new entry is copy of some previous, otherwise it returns an error (If you don't have such index, there will be a risk that you enter the batch of classes several times, unless you add a WHERE clause to check for double entries into append query). When the query was successful, run the query qClearTempClasses - the temporary table will be empty for next batch of classes. Repeat the cycle until the table is filled