From Sweden but used to live in Thailand for many years and traveled to most of the Asian spots. Unfortunately I have to say that April is probably the worst month finding a reliable spot. This is when the Thai New Year is and we always tried to find a spot with consistent wind but never succeeded.
If you can go later May-Sep I would highly recommend Sri Lanka as these are really surf camps as there is nothing to do outside the surf camp and you eat breakfast lunch and dinner together at the camp. I went to this one and can highly recommend it https://www.kitesurfinglanka.com/
There will be a mix of people from all over the world.
If you can go earlier Oct-March I would really recommend Phan Rang in Vietnam, either https://www.phikiteschool.com/
I went to Phi:s center which is also highly recommended. I still think that there's a Swedish instructor at Phan Rang kitecenter.
I have not been to the spot on Sulawesi recommended earlier in this thread. It might have wind in April as they state but I wonder if it's really busy and that you will meet a lot of people there. Seems more like a resort than a camp.
Windwise in April I would say that Taiwan is the best option, apart from Sulawesi which I never visited, but there are not really any surf camps in Taiwan either.