Transport for London has been criticised by the Information Commissioner's Office for collecting data on child travellers "without a clear purpose", reports the BBC.
TfL collects names, ages, addresses and school details of children aged 11-18 who apply for free travel using the Oyster Zip photocard.
TfL admits application data will be shared with its subsidiaries. Those not wanting to share their information will not get a card and will not be able to travel for free.
TfL said it needed the information to prevent anti-social behaviour and to be able to cancel the card to prevent miscreants from travelling on its network.
Police already use information collected from the adult Oyster card system to glean details about passenger movements.
BBC London has learned that in the past year alone, they have made at least 3,000 requests for information.
A spokesperson for the ICO told the BBC, "It is not clear whether young people are really being offered a genuinely free choice here.
"We have some concerns that the information provided to young people about Zip cards does not clearly state why the personal information is being collected, how it will be used and how long it will be retained."
The ICO said it would be contacting the TfL about its concerns.
TfL said it needed the information to make sure young travellers abided its "behaviour code".