| | Re: on his students' notes/in
On or In is fine for the slides. If they were being projected on the wall, on a sreen I'd probably use "on", but "in" wouldn't be wrong. If they were printed out and distributed on paper then my preference would probably be "in".
However, with "notes" the preposition would definitely be "in".
On + physical object : on paper / on the slides / on the screen / on the wall / on the page
In = contained in the middle of some information : in his speech / in the summary .
Notes are information rather than a physical object so "in" must be used
Slides can be seen as physical "things" ie the container of the information - so "on", or as the information itself - so "in". The same with things like leaflets and handouts : You'll find the name somewhere in/on the leaflet/handout