Can I stay in Thailand until my Ed-Visa extension expires if my language program ends early?

Legally, when you applied for your visa extension you said you would notify immigration of any change in your status. Since your extension was obtained based on studying, if you stop studying you are required to notify immigration that you have ceased studying and your extension will be invalid as of the day your studying ends.

If you never plan to get another long-term visa or extension in Thailand again, then you can just stay until July and let it expire and leave. But if you do expect to get another long-term visa or extension again in the future, you need to cancel your education extension with paperwork from your school, and then leave before the last day of your studies.