Hey. I will be going to Immigration this week to cancel my ED visa (1st extension, so in the 2nd 90 days of it) which expires on the 10th of April.

Two questions:
1. I have the letter from my school + some other documents they gave me. Is this all I need? Any TM form or copies of passport or anything?
2. I will leave the country on the 31st of March, and the letter says to cancel the visa on April 1st. Is this correct - to cancel the visa the day after and not the day I'm leaving? Makes sense, but just making sure.

Thanks in advance guys! You're always so helpful <3

Are you planning to leave via Nong Khai to Vientane ? That is the only case in which you need to cancel your extension, as immigration their abuse their power not letting you leave without the extension cancelled.

Yes I am ^

Letter should be all you need to cancel.

As long as you don't leave after the date in the letter it should be fine, otherwise you are on overstay.