I had this as an adult - scared me until I talked to the doctor and learned that lymph nodes swell as part of the body's immune system. Totally normal and a good sign - the body is fighting something off. No different than a fever - immunity at work. Like a fever, you ignore it if it's low-grade and there are no other problems.
Does she have any other symptoms besides the sore throat? The throat could be due to her breathing but she could be fighting a virus. If she's running a high fever, if she's lethargic, if she's needing a lot more sleep (beyond regular teenager stuff), it COULD be a sign of something else. My son had mononucleosis in high school - sometimes that's a minor thing and sometimes it's major. (Ours was major but we found a way to really turn it around - but of course there is no medication for a virus so we had to do something else.)
I don't think lymph node swelling is more common in teens for any reason other than they get overtired, they go to bed too late, they cram in too many activities, and they are getting up for school way beyond what their body clocks are really set for (there were news reports on this just last week, about how high school ought to start at 8:30 a.m.!). So barring any major symptoms, I'd do what you always do when a kid is fighting something off -more fluids, more rest, better nutrition.