I know we got a thread listing all OSes in the help section, but I feel it would be nice to also have one here that lists only the Ndless-compatible OSes, so it's easier for people who are introduced to Ndless.

TI-Nspire
-OS 1.7.1.50
-OS 1.7.2741
-OS 2.0.1.60
-OS 2.1.0.631 (Requires Nleash to downgrade below 1.7 once installed. This OS is HUGE!)

TI-Nspire CAS:
-OS 1.7.1.50 (Not compatible with CAS Touchpad models, even with a clickpad on)
-OS 1.7.2741 (Not compatible with CAS Touchpad models, even with a clickpad on)
-OS 2.0.1.60
-OS 2.1.0.631 (Requires Nleash to downgrade below 1.7 once installed. This OS is HUGE!)

Those are links to TI server. Feel free to add other OS links from there if there are any other that are compatible.

i dont think you can open any ndless files in student software.
If you're upgrading from 2.0 to 2.1 (I suppose that's what you're trying to do), you'll have to get a different version of ndless.

If I remember well, OSes 1.7.1.xx and 1.7.2.xx don't work with Ndless 2.

You really need OS 1.7.2741.
OS 1.7.2733 works too but is not available on TI website to my knowledge.

I believe the highest OS you can use with ndless 3.1 is OS 3.1.0.392

Yeah TI removed most of the old files from their site, even softwares, so it's best to find them on TI-Planet for now (as long as TI won't send them a DMCA notice like they did in 2010 for OS 1.1)