TheyByzantine has an excellent suggestion. I'd maybe work on the other end too; don't you know when you have tests/work scheduled so you can look at a calendar and give a date you will be free for delivery? Maybe you can pick a normally lighter school day and have a policy like, "I only deliver on Tuesdays" or something?

Has anyone complained about your deliveries or are you thinking in your own head that they are not good? That makes a difference :-) You are probably doing quite fine, especially if you're getting repeat Avon business; it's not likely they'd come back if they didn't like your way of doing business?