Just want to my 2cts worth of "professional" advice
There is no perfect school in this world. The fact that the standard of your child's English Language has deteriorated is not a good indication that the school is not doing a good job in educating your kids. What is considered as a good school? The reputation? The many distinctions your child has attained in school? I am not so sure if that is a yardstick to see how good the school is.
I dont mean to offend anyone but I think it is pretty sad that many Singapore parents have associated good schools with good grades. Many neighbourhood schools have great teaching practices, compared to many of these reputed schools where the teachers may not necessarily be good in teaching anyway. I would love a school that does not only practice great teaching methods but also emphasizes on character education. I certainly do not want my child to be achieving distinctions but has a lousy character. I want a school that builds my child's character too.
I also want to say that it is also the parents' responsibility to educate their child too. If you find your child speaking non-standard English then I think it would be good to let teach them how to speak good English. Mee Toh is essentially a Chinese clan school and parents must be prepared that the standard of Chinese Language there would probablly be higher than English. I can assure you that if your child goes to Chinese High, he will speak better Mandarin than English. Then is it alright to be better in Mandarin than English just because he is in Chinese HIgh? Let's not practise double standards here.
Good education is not just about grades.