The school needs to follow the IEP/PBSP. If they are following it and something like this happens, the PBSP might need to be tweaked. If they are not following the IEP/PBSP then they are out of compliance with the IEP. The school can't just 'try their best'. They have to follow the IEP.
I remember my sp ed director saying there were no excuses for an IEP not being followed. I agree with this 100%.