Superb
Yes, JIS is a very good school, and yes, it has quite a high number of ESL students in her P1 this year, which is quite "exceptional". However, I don't think the school has been getting worse. Students in other years, including the current reception class, do speak very good English. This year is so "exceptional" because last year the reception class admitted too few students (and they all speak very fluent English), and so this year the P1 class, unlike previous years, could admit many new students. Among these new faces, if they are not native, their English levels are typically not up to the usual JIS P1 standard. As a result, the number of ESL students is high. I heard that some old P1 parents are not too happy. Nevertheless, the reception class this year is quite full and so the same problem probably won't happen again.
I don't have any benchmark to compare with but I found the facilities are excellent; the school is spacious, has an indoor swimming pool, a grass pitch, a covered playground, an indoor gym; students are really multinational; the PE programme is superb; the implementation of the IB PYP is pretty successeful; students are well behaved and look smart.
You don't have to believe me. Just call and make an appointment to visit the school.