I love the coloring and the character design here, the whole picture looks good in general. The only details I would correct here are hands. I think the hand holding a notebook should have longer fingers - they seem very tiny when they bend like this. Take a look at your own hand and notice that every finger "starts" on a different height and the bending points are not on the same line either (not sure if my explanations are clear, sorry). Anyway, study hands more (I know they are a real pain to draw, I really know T_T).
Also I'd like to encourage you to try different shading. Lighting a character from the front is quite an easy solution, but I think it's time for you to experiment with lights a little bit more! Check for yourself how much it can change: [link] My advice is while drawing a character draw a source of light next to it. An orb or something like that and draw arrows coming out of it so that you know where to put the lights. This might help a little, but you know my theory - the key to learn how the lights and shadows work is an observation of nature and people. But I'm also sure you can find some pretty good tutorials in the Internet