Can my sons extend their stay in Thailand if they leave the country before their visa extension ends?

Hi, my sons are currently back from university and their tourist extension lasts till the 23 of August, they will have to fly back to university abroad on the 13th of September. So we were planning on leaving the country and coming back to extend one more month. However my sons have plans on the 20th-28th of August and we would like to avoid flying out during those days. Will it be possible to fly before the visa extension ends on the 23rd for example on the 15th and get a new visa? Will it cancel the old one and get a new visa for the remainder of their stay till the 13 th of September? Thank you