Yes, that's a problem I have in fourth and fifth grade due to large class
sizes and mixed groups of gifted and slow learners in the same class. Many of
the slower students do need at least three or more periods to finish a
project. Without giving them the opportunity, they'll get a poor feeling for
art. They'll never get above an "S" for satisfactory on their grades because
they didn't get enough time to finish. By fifth grade, some have a distaste
for art because of this. I've been learning throughout the years to give one
day per every two weeks to finish up old projects. They have to check for
their names on projects, put finalizing touches such as paint, glitter, more
vivid colors, etc. I can't force them to complete a project, but the projects
are available for them to complete. The rest can free draw for the day. I know
it sounds chaotic, but if it can be done twice a month, it helps in grading.
You can always tell the parents, yes the time was available, and no Johnny
didn't get an exceptional grade in art, because he chose to free draw that day
rather than give last minute touches to his work.