In Chapter 12 of Tom Sawyer, Tom gets "sick". In my opinion, Tom is faking his sickness because he`s capable of doing much more. Like faking his own funeral. And I think he was "sick" when he went to school because he wanted to make Becky feel sorry for him. And when he realized Becky didn`t go to school he felt very bad. So, he continued with his faking. That`s my opinion.