extras at Eton? Like what kind of extras available? - as many as the housemaster will allow, based on your son's timetable, his workload, and your willingness to pay.
Roughly, how much you pay termly for extras and trip or any other activities? - how long is a piece of string...you can pay for extra squash lessons at £35 a pop, extra music lessons, a trip to Japan for £5k...it's perfectly possible to pay little to no extras as well if that's what you decide. Trips are not mandatory.
How children do their instruments practice every day for example are there enough piano for daily practice? - there are tonnes of pianos, my DSs house has three available in-house but the music schools have lots too. Your DS has to have enough motivation and desire to practice without parental pressure to actually do it. It's not a spoon feeding hand holdy place for things like music practice. DS1 has an ME and how much he practices is his business - it's up to him to face his teachers and the rest of his ensembles if he hasn't practiced. If he makes a fool of himself by not practicing, that's on him.
I heard that some children do online tutoring during the evening or weekend. Do children have enough time to do extra lessons? - I don't know of anyone in DS1's block who has tutoring online during the evenings - there isn't any time anyway. Same for weekends in the half, there is far too much going on between ensembles, play rehearsals, sport, societies, house stuff, EW, etc. No idea what parents are doing on B weekends or leaves but from what I hear from DSs friends parents, none of them are being tutored then either, they're all sleeping, travelling, going to GB rowing camps, working at the pub, going to parties, or doing internships. I do think that if you have to be tutored during the half just to keep up, it's probably not the right school.
How many sets are there for the lessons especially for science and maths? Is science strong at Eton? - Not every subject is streamed. I think maths has 12 divisions but multiple divisions will be on a similar trajectory and they all do the same curriculum. I don't know how you define "strong", plenty of boys seem to go on to do STEM subjects at A-level and university and win all sorts of prizes and get papers published, we're always getting emails and social media posts about it, there are science prizes within the school, STEM professionals coming to speak to the boys in societies all the time...
If a child is above from a subject how they get support from the school? - I think you mean if they're falling behind in a subject? The division master will flag them to the housemaster, who will discuss with the boy what's needed - extra support from the div master, learning support for organisation or SEN, extra work or practice. However it's up to the boy to actually do the work or face the natural consequences. It does prepare them very well for having to work independently at university and beyond.
If you want the Eton result, you have to trust the process and buy into the Eton ethos which is very much an unashamedly big, busy, elite, full-boarding, academic environment where the boys are expected to manage themselves from day one without their parents steamrollering ahead of them. It definitely isn't for everyone - there's no shame in that, my DNephew wouldn't have coped, and went to another school and flourished there. But for boys who want to rise to the challenge, it's a brilliant environment where everything is possible and every resource is available to them.