@Serge ********
, that is an extension of stay stamp and in theory at least you should go cancel it before you leave the country (although not canceling it will not affect you getting a tourist visa from a thai consulate or a new Non-ED visa)
There could be problems with the extension of stay on the new ED visa though because you never canceled this one.
I would say take a letter from the school stating the child stops studying on the date you're going to leave the country, have the immigration office cancel this extension and stamp them in until that date and then leave the country on that date <- that is how you officially cancel and extension of stay.