This was probably a bug, see similar issues such as Vector toolbar contains no tools and Snapping Toolbar missing from View > Toolbars menu.
QGIS 3.2 was a minor release version of the program. For teaching a class, I recommend using the long term release version (LTR). The LTR is much more stable, and will have bug fixes applied even after the next minor releases come out.
Minor release versions have some serious drawbacks.
The biggest drawback is bugs - sometimes minor, sometimes huge. I've seen minor releases where most of the vector analysis gave incorrect results. Imagine if that happened while you're trying to teach a class. You'd reasonably assume your students were doing something wrong, and you'd waste a lot of time trying to correct their "mistakes." The students get frustrated, lose trust in your expertise, and don't learn very much.
New features can be confusing to students. Imagine trying to follow a tutorial with step-by-step instructions and screenshots, but the program on your screen doesn't match. Features have different names, buttons are on different toolbars, icons are different colors, the expression builder and Python Console use different syntax.
New minor versions of QGIS come out every few months. This means your teaching materials will go obsolete. If you teach the class again, you'll have to walk through every lesson looking for things that might have changed.
Don't teach a class using a minor release version of QGIS. Use the long-term release. You'll save yourself a lot of work, and save your students a lot of frustration.