For school I have to buy a new calculator with a CAS system. I thought of buying one from Texas Instruments. Any recommendations for the specific model?

TI-Nspire CX Ⅱ CAS

I also hear great things about the TI-89

Only the Nspire CX II CAS is still maintained (receives updates etc) and it's the one I'd recommend considering it's modern and way more capable than the 89T, too.

Here is a comparison of all the CAS calcs from TI:
https://tiplanet.org/forum/compare.php?89t,89,89hw1,92,92ii,92p,v200,nscas,nscastp,nscxcascr4,nscx2cas

I believe you should get the new TI–84 Plus CE Python, if for school. If not go to the TI–Nspire CX II CAS.

You should not get the TI-84+CE Python. It is slow, has limited programming capabilities, and the embedded python is lacking in many areas. Nor does it have a CAS. However, the normal TI-84+CE is a fine calculator.

The TI-84+CE Python is strictly better than a CE of the same hardware revision and OS version without Python, even if the terrible Python implementation means it's only barely so. Definitely don't pay more for a regular CE over a Python one, but also don't pay more for a Python CE over a regular one, unless you're in a class that requires it.

That being said, a non-Python CE might have an older OS version that still supports assembly, whereas any Python CE won't support assembly. That's not particularly relevant considering that you can still use arTIfiCE to run assembly programs, though.

The Nspire CX II CAS is a great calculator for school and you won't regret getting it. However, the HP Prime G2 is faster, has more storage, also has Python and CAS, and has a fantastic touch screen rather than the touch pad. I personally prefer it and it's definitely a calculator you should look into.

I think the cx ii is faster than the prime g2 in some overclocked benchmarks?

Maybe, but at this point it doesn't really matter, it's plenty fast enough.

Anyway, when you have a TI calculator, you also get a huge amount of programs available, and a big community to help you. Not much exists for the HP Prime...

It might in a few specific tests (It didn't in any of the tests documented by TI-Planet), but overall the HP Prime is still faster than an overclocked Nspire CX II. Besides, the overclock requires a jailbreak to overclock so you're not getting any of that extra speed in Exam Mode. Also, you can't get more RAM/storage through software so the Prime still wins there as well

is there a speed difference between the TI-84 Plus CE Python and the normal CE?

Not if they are both hw rev M or later. Pre-M will be slower. But are guaranteed, when bought new, to have an OS that doesn't have ASM blocked.

What are the speed differences between a pre rev-m and after rev-m CE?

It's pretty big, TLM made a thread or a video or something of the matter iirc...