Yeah move your neck around and you could try using ice packs on it , Heat makes bruising more worse than before
You should take care of it ,like sleep with nice pillow,and do some neck exercise.
Maybe your bookbag is too heavy. (mine probably is) or maybe you slept on it wrong.
Did you sleep wrong? I hate when that happens xD Just do some neck exercises? XD Message it. Take a bath :D It should feel better in a few days. Just don't sleep with a pillow tonight.