After his body shrank, Conan attends school in order to look like a normal child especially in front of Ran and Kogoro, but why does Haibara attend school? Why does not she dedicate her time trying to redevelop the APTX drug and trying to find an antibody to it?
Haibara is still in the form of a child and is not hidden from society, so people know who she is. If she doesn't go to school, people will question what is going on, and she can probably keep better tabs on Conan and make sure he doesn't accidentally reveal his identity.
According to Detective Conan World Wiki,
Because she occasionally works overnight on the antidote for the APTX 4869, she tends to be tired quite frequently, causing Conan to call her an "evil-eyed yawny girl" on one occasion.
So, it is likely that school can also function as a sort of break for her. Also, it's not like she isn't working on it a lot. The files on the drug were all deleted so it is difficult for her to create a permanent antidote and as such, makes progress slow on it.