Disassembly of Reason

 I tried Googling and got general info on curriculum, specific schools, and alumnae pages, but not the specific details I'm looking for.

My main character is an upper class 13-year-old boy who, due to long-term illness/frailty was educated by tutors at home, but is now being sent away to school. (Probably England, but since he lives in Yorkshire, I'm not completely opposed to it being Scotland if it turns out that there are significant differences in school experiences between the two countries and Scotland would work better for my character.)

1. Would most pupils begin classes in the fall term as is typical today? As in, would it be realistic for him to begin in September? Or was it more of a rolling admissions thing?

2. How far in advance would his father need to contact the school to enrol/register him? Would there normally be an entrance exam, or would it normally be, 'We accept anyone who can pay the fee, provided there's space'?

3. When it comes to letters to and from home, would there be any reasonable expectation of privacy, or would it be common for staff to read each piece of correspondence? Specifically, the boy has a female cousin of the same age with whom he's quite close. Would he get into any sort of trouble for writing to her or would her letters to him be delivered?

4. If she were in the vicinity of the school, would it be possible for them to meet under the auspices of a chaperone, or would that be totally unheard of?

5. How much would the outbreak of WWI impact him? I would guess that some of the younger teachers would have enlisted over the summer and some of the older boys talking about hoping it won't be over before they get a chance to get in it. Would that mean larger than expected classes? 

Thanks so much!