I've done the JMSS entrance exam in 2016 and attending JMSS. I don't remember much but I think it is important to have a go at as many questions as possible in 30 minutes.
For multiple choice ones, if you cannot answer a question immediately just skip to next question and attept other easier ones first and then do the time consuming ones. At the end, right before the teacher tell you to put your pen down, just guess any random answer for question you couldn't answer. You would get at least one of them right.
If you've done some practice exams you will be able to do it. The one I remember clearly is that for the science writing, there were two of them and one of them was about writing a scientific report. I think it is worth knowing basic thing about procedure, result, conclusion etc. It is okay if you don't know (I didn't know about them and still got in). The other science writing was just reading an article and kinda summarising it (the article was about coral bleaching).
You don't need to get 100%. Even myself with bad english got in. If someone's worrying about their English or doing EAL, you don't really need to worry about it.
Hope this helps! Good luck everyone for their entrance exam:D