My advice is to first explain to them that writing takes time, and you won't be able to finish part two overnight. After all, chapter books take months to finish. Tell your friends to be patient, and you'll have part two out for them to soon as soon as you possibly can. What with end of the year testing and whatnot, I'm sure your classmates have a ton on their hands, too. Just make sure they understand that you won't have it out right away, so at least you won't be bothered by them.
In my opinion, the best time for you to work on part two of your story is when you have some extra time, like over the weekend, or on weekdays when all of your homework is done. If you're writing it on paper, you could always work on your story at recess or lunch, unless you don't want to sit around writing during lunch. It's your choice when you work on you story, but do it at a time when it's most convenient for you. Considering that your friends are eager to read your story, you shouldn't put off writing it, but don't do it before your homework. Write it when you have down time, or on homework-free nights. If you have testing this week or next week, you shouldn't have a lot of homework assignments (besides studying), so that would be a good time to write part two of your story. c;