For India generally you have to deal with a company called VFS, you pay them they lose your documents and your Visa is void because you haven't supplied the necessary documentation. But of course they keep your money.
You really should specify where you want to stay in Thailand because each school is connected to a province and you generally can't get a Visa in another province to where you live.
as another experiment I have not taken my passport out of my drawer except for immigration visits, work permit, social department and opening my bank account last week for the past 3 years. Never needed it for traveling or anything, just use my licence.
A re-entry is valid for as long as the visa is valid.
With most education visas that means every 3 months you get a visa extension.
So that means your Visa is expiring every 3 months.
You can arrange to get your visa up to one month early.
That gives you 4 months to be away if it's a desperate situation and you want to keep your Visa before your next extension.
I am thinking that you're in your first or second visa extension, because if you're in your third visa extension you wouldn't be asking this question you would just leave early.