I have used Microsoft OneNote 2016 (plus screensharing software) for online tutorial sessions in number theory during the past year. It has worked very well and I will continue to use it.
You can enter plain text just like in Microsoft Word and switch to Math mode by pressing a shortcut (["Alt" + "+"] in my case (German), but it might also be ["Alt" + "="] or something else). In Math mode, you can essentially write LaTeX code which is transformed into mathematics upon pressing space. (This works similarly to autocorrection.) Some experience is required to use this swiftly and there are some differences to LaTeX (e.g. round brackets are used for logical grouping, not curly brackets). It is also not as powerful as LaTeX, but you can do all kinds of basic mathematics like greek letters, basic relational symbols, integrals, summation symbols, matrices etc. You can also add, remove and change math commands (go to Options>Proofing>Autocorrect options). For example, I added lots of LaTeX commands without backslashes for myself, so I can write "alpha" instead of "\alpha" to get $\alpha$.
An example from my tutorial sessions:
The formatting is not as great as in LaTeX, obviously.
- The same math functionality is available in Microsoft Word, but OneNote is free and supports infinitely large pages. Further, if you have a touchscreen device with a pen, you can also make drawings next to your text as needed.
- Make sure you use OneNote 2016, not OneNote for Windows 10. These are two similar versions of OneNote (both are freely available), but OneNote 2016 is better suited for this usage.