The issue of schools not taking photos is not as simple as 'they simply can't do it without permission'.
If you take photos of individual children for use on ID badges, this would constitute personal information under the Data Protection Act 1998 and would thus require consent from parents. However, if you were to take a photo of a group of children engaged in a school activitiy and print it in a school promotion leaflet, that would not constitute personal info (unless the children were named individually).
The idea is that organisations that hold personal data about individuals must ensure that they use it fairly, keep it secure, make sure the information is accurate and keep it up to date. Individuals have the right to view the information held about them (subject to a fee of �10) and have it amended where inaccurate, or destroyed where it breaches the DPA.
With audio recordings, it will depend very much on how you store and use the information as to whether it comes under the Act. If you record individual children, and store those recordings in such a way that those individuals can be identified, it may be that the DPA applies. It's a difficult one, since voices are somewhere ephemeral; it isn't easy to imagine many ways in which that information would be likely to breach the rights of the individual child, but I would be tempted to err on the side of caution.
If you seek permission, you know you're covered - so long as you meet the other obligations - e.g. to store it securely etc.