My dd10 had 19 kids in her 4th grade class. Due to budget cuts, they reduced from four classes to three this year and they added 10+ kids to the grade. That led to all of the 5th grade classes having btwn 30-32 students this year.
My dd, like yours, has ADD, is not highly motivated, and gets overlooked in large classes b/c she doesn't cause problems. However, she underachieves, but not enough to draw attention b/c, for her, underachievement is performing like an average - high average kid.
The only way this year was tenable was due to a GT teacher who is something like .6 FTE. She works only with the 4th and 5th graders and dd goes to her class every day for both math and reading. So, she is with a smaller group of about 15-20 kids for both of those subjects. It would take a really, really good teacher with some creative approaches and a lot of parent volunteers IMHO to make a mixed ability grouping of 30+ kids work for a 2e kid.
