To do this you can create multiple formulas in the Grade Plan so that teachers can pick which Grade Plan formula to use when Calculating Grades.
1. Go to onRecord > Settings > Grading.
3. Select School Year Setup.
4. Set any needed filters to locate the Grade Plan and click View.
5. Click View next to the Grade Plan to edit.
6. Click Edit.
7. Click the Arrow to the left of the Formula for the Grade.
8. Add a Single Term Formula (or Multi if you use Multi Term only), adjusting the percentages as needed to add to 100%.
9. Click Close.
10. Do the same for the Multiterm Formula if your School Uses Multi Term.
12. Click Save & Exit.
This will now let teachers choose a formula, either one with a Semester 1 exam or one without (as per how you named it in the steps above). You will want to do this for each Semester that would have an exam in it, so that teachers have the option of exempting from it, for example if you have a Semester 2 exam as well.