I am looking to upgrade from a Canon Rebel xTi camera to a newer mirrorless camera. Looking for two things: 1. advice on a new, similar, beginner camera (that high school students could use) AND 2. will the EF-s 18-55 mm lens that I already own work with a new camera or do I need an adapter/new lens?
Tough question to answer without more info. What sort of budget per camera are you looking at? When does the camera need to be delivered?
You will most likely be able to use your current lens with any of the Canon M Series or R Series bodies, but I think you will be let down by the performance and end results. Sell the lens with the old camera, or keep them both as a keepsake.
However, the rumor mill has it that Canon may be announcing some entry new entry level mirrorless camera bodies. Canon has also just announced current M Series bodies will not longer be produced.
In other words, with chip shortages, shipping delays, and new camera announcements everything is in state of flux.
Generally agree with Waddizzle.
It will help us a lot to know a bunch of things:
1. What is the budget in general terms?
2. What kinds of images do you want to shoot? General social, wildlife, portraits? Each has different demands on a camera. The idea that you seemed to be happy with your 18-55 lens suggest that something in the general purpose line may work for you.
3. What do you intend to produce? Do you, do very large, high-quality prints, smaller prints to frame, or for digital display and social media - that makes a big difference.
4. What is your intent with the camera in the future. What I am getting at is, if you are not needing to engage with constant upgrades, then you could take advantage of the good DSLRs that are now on the market at good prices.
There are two general lines of thought that might work for you:
a. As said, wait for a while and go for an entry-level mirrorless camera and an appropriate kit lens.
b. If you are not serious about photography and what you have will last a while, take advantage of the large amount of gear that is being sold by enthusiasts as they invest in mirrorless cameras. There are lots of highly-capable DSLRs out there, and some relatively high-end stuff is going at some really cheap prices. You will be able to get much better bodies and lenses than you have now if you head in that direction. A good camera will always be a good camera and the legacy DSLR gear will still be brilliant for students.
In general terms you might want to consider the impact of your high school student. A good quality camera, something in the 80D or 90D range will be better built, have better performance and will work well for a student. In terms of lenses for such a camera, a 18-135 IS STM or IS USM lens would be a great walk-around and general-purpose lens. As was indicated your old 18-55 lens is obsolescent, to be honest. Your existing camera could work well as a very beginner kit for your student and then they can progress to the new DSLR with seamlessly.
If they are going to play sports and you want a telephoto lens, get the EF 70-300 IS USM MkII lens, it's brilliant and very effective as a wildlife and sports lens. You might see some of those on the market too.
If you are determined to go mirrorless, you could at Canon's refurbishment site and see what they have - they are like new and come with a 1-year warranty. The Canon EOS RP is a good starter camera and there are usually some refurbished units available. On the lens side, the RF 24-240 IS USM is actually a great lens and hold its own against the high end RF 24-105 L series with a comparable range and would provide an all-in-one solution. Again, much depends on your budget and if you are prepared to wait for the rumoured consumer-grade RF cameras to be delivered, as opposed to being announced - which may take a while...