I paid $400 to my children's school in lieu of volunteering 40 hours; is that money tax deductible?

The IRS allows you to claim a deduction for the donations you make to qualified organizations. These organizations include more than just charities and will include any school district program that does not operate for profit and is solely supported by state and local governments. When you fill out your tax return this year, you can include all contributions you make to these schools as a deductible charitable contribution.

Amounts given directly to a for-profit school are not deductible. If the institution operates a qualified charity, donations to that would be deductible.

See this article for more information.

Q. I paid $400 to my children's school in lieu of volunteering 40 hours; is that money tax deductible?

A.  No, because you receive a personal benefit from the payment. 

Even if the school is a recognized charity, you cannot claim a deduction.  Essentially, your cost of attendance, in addition to regular tuition,  is either is $400 or 40 hours of volunteering.  Choosing the cash option doesn't make it a charitable contribution.  It's still the cost of sending your children to the school.