Is it considered concealing knowledge for a student to keep their personal summaries and notes to themselves, not sharing them with fellow students for fear that the latter might achieve better grades? And what is the ruling on that?

You are not religiously obligated to give your colleagues the summarized material, nor are you considered to be withholding knowledge. However, if you help them with it, seeking the countenance of Allah, you will be rewarded. And if they surpass you in worldly success, you may precede them in the ranks of the Hereafter, which is the true arena of competition.