Low light sports photography is demanding of the camera body, lens, and photographer and all three have to work together. As to hardware, with any decent modern camera body the lens is the most important part. Full frame sensors with a reasonable number of pixels (bigger photosites provide better light use with less noise) have an advantage but any of the recent decent quality cameras will do at least passable work with the right lens.
Unfortunately, the "right" lens can get expensive especially if you are trying to capture action far away and if you try to crop excessively with a reasonable cost "normal" telephoto lens then you will increase apparent noise and loss of detail.
IF you are thinking about an upgrade, first tell us what lens you are using and that is probably going to be the most important factor. I use 1DX III bodies for sports and they handle low light very well but if I switched from my f2.8 aperture glass to a slow f5.6 or worse lens then an entry level camera with a f2.8 lens would provide better results than a 1 series body with the wrong lens.
Make the most of what you currently have by:
1. Shoot in RAW instead of JPG which will allow you to get the most performance your camera lens and sensor is capable of providing and this also allows you to correct a lot of problems in post after the event.
2. I always shoot sports in manual mode with ISO set to auto. For sports, set your aperture to the widest (lowest number) F stop your lens provides. Set the shutter speed fast enough to avoid motion blur and for most high school sports 1/640 is workable although there will be some blurring with some action. I stay at 1/1,000 or faster which is preferable but depending upon your lens and sensor then 1/640 may be necessary to stay within the sensor ISO capabilities of your R-100. You can try 1/500 or even 1/400 in really poorly illuminated venues but you will get a LOT of motion blur with action shots at this speed. I much prefer noise over blur, you can reduce noise in post but blur is forever and is to be avoided when possible.
3. The camera is a system and you are a critical part of it. Choose your event location so that with your camera lens you can fill as much of the sensor as possible with typical action captures. This will provide the best performance the camera and lens can provide. Very few venues are evenly illuminated so if you are running into low light issues, concentrate on the action in the best illuminated areas. Ultimately, spend your time on the captures your camera system can capture and don't waste time (ultimately missing other photo ops) trying to take images beyond those capabilities.
4. Don't constantly check your just captured images. Distraction on the sidelines is a danger to the student athletes and yourself and this will also cause you to miss photo ops. Players are focused when playing, you are on their turf so respect them by staying focused on the ongoing game and not your camera. The officials must NEVER be inconvenienced by your photography work. I have been doing sports photography in the area for years and know most of the crews but when new people show up, I make sure to introduce myself when they aren't busy prior to the start of the event. And officials rarely get good action photos of themselves and most of them appreciate these so take good care of the officials and they will take care of you.
Ultimately, it is a learning process and at the beginning the learning curve is steep so prepare to learn a lot from images that didn't turn out as planned. Also keep in mind that as photographers we tend to be critical of our work. By profession, I have a PhD in marketing and I spent much of my career also doing consulting in risk and quality management so I tend to be highly attuned to quality. But an image that has lots of flaws to the eye of the photographer can be a beautiful image to an athlete.
I will never forget one image I took several years ago at a horribly illuminated football field where the already horrible lighting got worse when two light standards went dark just before the ball was snapped. I was out of position because it was third and a short 1 for the opposing team and I expected a run play but the quarterback dropped back and attempted a long pass. It was intercepted and I captured that interception but the lighting pushed my 1DX II to ISO 40,000 and I had to do significant cropping also. To me it was a very noisy poor quality photo that I considered not using but the player who made the interception went out with a season ending concussion later that game and he and his family loved that photo and had it made into a poster for his room. So be critical but not TOO critical, your goal is continuous quality improvement but it doesn't mean you won't get some very nice images along the learning curve. The second example image is at ISO 20,000, which is still fairly high, but shot with a 400 f2.8 that required less cropping and with a 1DX III that provides slightly better high ISO performance than the 1DX II used for the prior photo. NEITHER would be something you would use for a studio portrait where you control the lighting and pose so except those rare occasions when the gods of light and luck are both smiling upon you for a rare instant, don't expect studio quality portraits shooting at sports events. Of course you can still get some nice expressions even in low light with high ISO like the last example of a very emotionally charged player on the sidelines which was captured with a 1DX III and EF 400 f2.8 @ ISO 16,000. I grabbed this just before the snap; maintain situational awareness at the game and you will find a lot of good photo opps off the field during brief lulls in game action.
Rodger
