If the person isnt willing to put in the effort during those 3 months, just attending the course will not get him anywhere, right? So let me sum up my high school years for you. You need to understand that while the coursework was in english and the teachers taught in english, the use of the english language itself wasnt enforced as a law in the school. Same is the case for many other international schools across Saudi Arabia.
As I stated above, I studied in Saudi Arabia. It was an international school. We were a batch of 25 in total and when you would enter the class, it would become apparent that 10 were Arabs, 10 were Pakistanis/Indians and there was this small group of 5 that had both. They always used to sit like that in groups. This is not something unusual. I go to my university dining hall and I see a similar picture. Internationals with internationals, and locals with locals. This is the very first downfall in my opinion. Because when sitting with your own community, one tends to speak the native language.
So suppose you are sitting like that and want a pencil, or want to understand a problem. You will turn to the guy next to you and speak in your native language. Because it just tends to make things so much easier. Then come informal discussions or lunch breaks, they use their native language. And out of the school is 101% native language.
Now coming to academics, apart from the period of english, other suybjects just tend to build up the ability to read in english. Writing doesnt improve because you can literally write half sentences or miss a period at the end, but if your solving is correct, you get full marks. Because the main focus of other subjecvts is to focus on the formulas/concepts. Coming to the english period, most students just aimed for a B or a C+ because in numbers, its a solid 70 or 75 so its OK. Basic idea and run-on sentences did get them that atleastat at my school and at the Olevel (Grade 10) examinations. After grade 10, CIE is a lot more flexible. You have the option of not studying english at all. We still had an english period in Grade 11 and 12 but we just repeated what we did in Grade 9 and 10.