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.
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