Meh, you’re not going to hurt a modern manual with a learner. They’re not even likely to hurt the clutch.

Internals if a transmission are primarily made of 3 materials: brass, aluminum, and *hardened steel".

Aluminum is for shift forks, I can’t even imagine a way to break one.

Brass is for synchros, which can be worn by grinding - which isn’t really easy to make happen anymore, plus but it’s not like you’re gonna sit there and hold it while it’s grinding, you’ll release it quickly. The last car I remember having grinding issues was because they didn’t use a synchro for second gear so you got a short little growl if you didn’t shift “just so”. Last time I drove that car it had 250k on the odometer.

I’ve seen dragsters miss-shift on 1960’s gearboxes that weren’t built for 450hp/500lb torque, and they’ve survived it fine (I’ve also seen them fail the same way). A new driver in today’s cars just can’t do that kind of damage unless it’s intentional - and that would take some time.