Regression in toilet training in my 1st grade sone

My only child (son) did fine through his 1st grade school yr. with toilet
habits until early March when I also went back to work full-time. I had
worked part-time prior to that, but from home. I drop off my son at school
in the morning and my husband picks him up from school. My son has behaved
pretty much as if this new situation is a non-issue, since it began. Other
factors that may be at play are occasional, unruly but harmless behavior by
his peers when they are all in the bathroom at once, AND one time when one
older boy accidently walked in on my son when he was in the stall using the
toilet. According to my son, though, once the boy peeked in and saw that
someone was in there, he just ducked out, and this happened just the one
time. My son's teacher is a wonderful woman. When she tells me that she
allows my son to go to the bathroom WHENEVER he asks, I believe her. First,
my son only had "accidents" during school time, not at home. Then the
problem grew to occasional home accidents. He has seen our pediatrician who
prescribed a stool softener last Friday, thinking this will help my son
"go," when he chooses to make the trip to the toilet. We give him the med.
at 6pm each night. Three days ago, we resorted to sending him to school in
pull-ups. We told him it was not punishment, just what we need to do now,
because we can't expect our (caring, patient) school staff to keep cleaning
BM accidents from underwear. He did not want to wear the pull-ups but he's
been pretty compliant about it. We have tried using various reward systems,
and this obviously has not helped. Suggestions, anyone???? Thanks. --Mary