You do not legally need consent to put anyone on for "bona fide news"
However, if they let you inside the school, many schools themselves ask you to not show faces. That's not a law, that's just school's request.
*BUT* there's another side to this. If you are doing a story on a school closing, you'd be able to show kids up the wazoo because it's just generic. If you are doing a story about some kid bringing a gun to school and the reporter says "A 16 year old male brought a 9mm to class..." and you are showing the face of some kid in a VO, you just linked that kid to a crime they didn't commit and legal action can be brought against your station and they will most likely be paying some money.
This is true for all stories/subjects, but children are offered even more protection.
If a news station is permitted access to an amusement park to do a story, everyone is fair game unless stated otherwise.
In saying this, I'm assuming you are a news shooter working for a news operation. If you aren't taking pictures for private use (family photos, etc.) then (this is most states from what I understand) you are not required to get permission in a "public area" as long as you are working on a "bona fide newscast, news interview, news documentary, or on the spot coverage of a bona fide news event."
Everything else requires permission if the person's featured in a product. That's why commercial shows (COPS when it was in production, any MOS-TV show deal, Girls Gone Wild) are required to get people to sign those forms. No permission, they have the right to not have their image be sold...
The reason news doesn't have to worry about this is that we aren't selling a product of that person and directly making money off of their image. That's the basic explaination.