Rather high risk for Year 10 as a totally new environment and also needs to prepare for GCSE. All are new for us.
So, I'm thinking about is it a good timing to study Year 10 in Sep 2017 as not prepare well. Though my son seems eager to go
Y10 and Y11 are the GCSE years whereby the students will study the GCSE syllabus throughout these 2 years for taking the GCSE exam at the end of Y11. Schools will generally not prefer to take in new students in the middle of these 2 years.
In fact, you can still find schools for Sep 2017 intake now, though some of the most sought-after schools may have already closed their registration book. It will be easier to find a school now to take your child for Sep 2017 than for taking him/her in the middle of the school year for Y10.