Most of what I was going to say has been covered pretty well already. I was diagnosed with Type 1 Diabetes when I was 14. My honeymoon period (that period of time when I was still producing some insulin, but not enough and not consistently) lasted for a very long time. My symptoms started 1 year before the diagnosis and my honeymoon period lasted until 4 months after it.
While there is a lot of controversy in the diabetic community over this (frequent Type 1 Type 2 showdowns happen on this topic online all the time), Type 2 is associated with obesity, but not necessarily caused by it. It is also much more common in adults (though that trend has been changing in recent years due to the growing waistlines of children). So either way Type 2 would be more unlikely than Type 1 for a 16 year old Cheerleader. Lastly, Type 1 is also known as Juvenile Diabetes (though adults can be diagnosed with it well into adulthood and in that case the name changes to Adult Onset Juvenile Diabetes).
Some other things that I thought you might need to know for your character.
There are 2 daily difficulties with Type 1 diabetes that are constant: High Blood sugar and Low blood sugar. Low B.S. is a more immediate threat because it can lead to Diabetic comma, seizures, and death in the short run and in the long run it can affect brain function (specifically memory loss) if extreme cases are very frequent. I'm actually struggling with this right now because I spontaneously developed something called the Dawn Phenomenon (where your Blood Sugar drops drastically for no apparent reason close to dawn). For a period of about a month I was going so low in my sleep that I was having seizures and not even knowing about it). Whereas my memory used to be extremely good, now I'm constantly forgetting what I was about to say . . . case and point is this post which so far as resulted in me going on an irrelevant tangent twice already instead of staying on point . . . sorry.
High blood sugar can lead to DKA (as mentioned earlier in another post), and that can lead to death (the only grand short term adverse reaction). In the long run it leads to all those complications people associate with Diabetes (blindness, loss of kidney function, amputation, heart attacks, strokes, and so on).
I only bring this up because if your character is out of her honeymoon period and is experiencing all the symptoms of High Blood sugar, then passing out would be a catastrophic event. You don't pass out from High Blood sugar unless you have been at record high numbers for a very long time. She would also have gone through so many other issues before the passing out stage that it would be impossible for her to function in school or elsewhere in any capacity. Trust me, I've been there and for 24 hours I looked, felt and acted like a zombie from a cheesy horror movie. The sequence would go as such: Increased thirstiness to the point of madness (I'm talking about chugging water nearly nonstop), increased urination, extreme fatigue, drastic changing in mood/behavior/personality, gradual darkening and sinking of the eyes, vomiting, and eventually collapse (though she would have been bedridden long before the actual collapse).
Low Blood sugar is different. I won't list all the symptoms as they are readily available online, but one main difference is that it can be extremely sudden and it can lead to passing out fairly easily and quickly.
As a cheerleader she would have issues with keeping her Blood sugar under control (mainly keeping it up) since physical activity lowers blood sugar. Another issue is weight gain. Insulin (an absolute necessity for Type 1 diabetics) is also a growth hormone (so it can lead to mild weight gain even under normal use). On the other hand, keeping your Blood Sugar high (but not too high) all the time leads to rapid weight loss. These are issues constantly discussed on Diabetic Forums online. There are actually several eating disorders specific to diabetics (teens specifically), but I'm not as educated on them as I never struggled with any of those.
For extra punishment and torture you can make you character a Brittle Diabetic (a term and condition that is still debated in the medical community). Brittle diabetics as in constant diabetic hell because their pancreas (which produces insulin) sometimes works normally, sometimes not at all, and sometimes goes into overdrive (producing far too much insulin for no reason at all). While many will say it doesn't exist and tend to blame the diabetic for practicing poor control, I have actually seen it in action and attended the funeral of such a person who died from it. It's quite spectacular (in a morbid kind of way) to see a woman eat 10 times the amount of glucose tablets (the best treatment for low blood sugar) need to treat a low blood sugar episode, wash them down with a whole jug of juice (also very good for treating a low), and finish it off with a few spoonfuls of honey (fantastic for treating a low) and 30 minutes later test her sugar and see it in the 30s (with normal being between 80 and 120) even though she has no insulin in her system. I've also seen her take absurd amounts of insulin for something as little as a piece of bread and still test off the charts High.
Anyway, now that I've probable bored you with a whole lot of information you probably don't care about I'll leave you with this: one of the best online forums for diabetics is called tudiabetes.org. The people there are very friendly and will be more than happy to help you with any info you need. And with more than 15,000 members you are sure to find an answer to your questions.
Sorry for the long post, but this was a good way for me to warm up my fingers for the writing I'm about to do as I start working on my WIP again.