A very good spot. Feel free to add it to the continuity error page.

It looks like she was just added to the chat as there is an encrypiton message which is what you get if you have just been added. Matty is a fairly common name too. Theyve made it known lily is only 12 so i think the Year 10 thing was her being added so they could get the truth and the Matty was a considence, it's highly unlikely Matty would also go to the same school after having an altercation with Tommy he was most likely expelled from primary school and not allowed to go to the same school as Tommy