Not being clear about when we use modal windows will result in an inconsistent and seemingly random UX across the site.
Guideline 1
Use a modal when there is a "happy day" scenario that needs to be interrupted. For example, log in/sign up to access a feature. This is a modal because it "steps in" if you are not logged in after clicking on something. Another example would be interstitials on landing pages asking the teacher to provide some information before proceeding. In each case, the teacher can see the "target" page behind the modal, reassuring them that they will get there as soon as the business of the modal is done with.
Keeping things in page (in expanding panels if need be) is better than modals because the teacher is free to consider other elements of the page at all times. This makes them feel more in control - they don't feel corralled into doing things.
NOTE: Recent observations in user testing have shown that the overlay on login/signup on mobile often causes people to scroll the background by mistake because people scroll in the gutter beside them. Overlays on mobile should therefore not be used when they cover so much of the screen that they may as well be separate pages.
Guideline 2
Use modals for "question mark" help text. This follows a loose convention elsewhere in which help text appears in a separate window. As such it's a different case where we follow convention rather than establish our own rule.