On the VBA menu >> Tools >> Options >> Editor tab, check all the boxes in the Code Settings frame. On the General tab, check Show Tool Tips, Notify before State Loss, Break on all Errors, and Compile on Demand.

Click OK to exit that.

In the VBA Editor, on the left side of the code page, there is a vertical bar, the same color as the menu bar. Left click that bar next to the Private Sub SchoolNameComboBox_Change. You just set a Breakpoint at that line. There should be a colored circle there, Click that circle to remove the Breakpoint

Start the UserForm and change the value of SchoolNameComboBox.
Now you should be looking at the Breakpointed line of code with that line highlighted, that highlight means that is the next line to be executed.

Hover the mouse pointer over ThisName in the line ThisName = SchoolNameComboBox. A Tool Tip should pop up saying something to the effect of ="". Don't touch the mouse, press F8 three times, watching the highlight move down to the ThisName = SchoolNameComboBox line. At this time that line has not been executed. Press F8 one more time, Now the Tool Tip for ThisName should show the school you selected in SchoolNameComboBox.

Now you have learned Breakpoints, F8, and Tooltips.

Press F8 until For i = LBound(SchoolNames) + 1 To UBound(SchoolNames) is executed.

Now hover the mouse over the i in the line For i = blah, blah. The Tooltip should be either 1 or 2 depending on some other settings. The first part of the line starting with If SchoolNames(i, 1) = ThisName Then should be highlighted. Hover over SchoolNames, then press F8. The Tooltip should read the first name in the list on the sheet. ThisName should always be the school name you selected in the Combobox.

Depending on the condition of the If...Then, the highlighted line will be either MaxDate = Application.Max(MaxDate, SchoolDays(i, 1)) or Next i.

In either case, set a Breakpoint at the Next i line. Press F5. this will make the code run until it hits a breakpoint, (Next i). IOW, it will run the loop once each time you press F5. Watch the Tooltip for SchoolNames and MaxDate. When SchoolNames = ThisName, MaxDate should change.

You have now learned Breakpoints, F5, F8, Tooltips, stepping thru code, and watching variable values.

Be sure to have a Breakpoint set at the line: NextAppDate = IIf(MaxDate = 0, "", MaxDate) so you know when the loop is done, else F5 will run thru the entire sub.