I agree with a pp that stated the issue may not be the breast feeding, but the fact that your child is attending. If you are getting free or reduced rates for his attendance, that may be the issue.
To say breast feeding isn't beneficial is ridiculous. But to play devil's advocate, I also don't know if this is affecting your work performance. Maybe it is more of an issue than you think?
In any case
and I'm sorry you are going through this: