My son just joined Year 1 at an international school. As he has dual nationality, the school offered us 2 options at registration - (1) register him in school as a foreign child (2) register him as Thai. The difference between the 2 was that if I registered him as a foreigner, he will only have 2 Thai lessons per week, but if he is registered as a Thai, he is required to have Thai lessons every day for 2 hrs/day, 5 days a week.
We chose to register him as a Thai, but the school did tell us that their Thai curriculum is a toned down version of that in Thai schools.