st06 hi!!! i didn't do JMSS myself but my close friend got in. she was also lacking in english and science, and what she used to do was borrow senior textbooks (usually bio, physics and chem) from the library and practice from them! all selective exams are always 1-2 years above the year level curricula, so working from senior textbooks always ensures that you have a strong grasp of the topics that will probably be on the test.
you can also make flashcards etc of these topics and practice daily in the morning, sort of like a routine.
for writing, that's something that comes with practice unfortunately. you just have to keep on writing. i also advice reading widely and frequently. your local library is an amazing resource with plenty of books.