I like to mix about 2-3 tbs. of peanut butter with a scoop of chocolate protein powder, then add just enough water to mix it up. Put it in tupperware or even a sandwich bag and take it to school with you. It's a great bulking treat that's low carb and perfect pre-workout in my opinion.
If you are not carb conscious, another simple treat is chocolate whey, oats, and a scoop of instant coffee. Keep it all dry in your shaker until time to mix it.