I haven't gone down this road yet.
2 reasons:
1st - school does not allow mobile phones for pupils at all (not at school, not on trips (even residentials) & definitely not at things like school discos.
2nd - I kind of thought it best to hold fire until going up to Year 7. The kind of thing to mark this right of passage.
By the way there's a nice discussion on the secondary discussion group about which mobile to buy Y7 & why here: www.mumsnet.com/Talk/secondary/a1816572-Which-mobile-for-new-y7-boy
I know that some of DD1's friends (DD1 about to start Y6) 'have mobiles' but when I've talked with the parents it's always turned out that they've been given a hand-me-down phone either from the parents or older siblings. When I've asked them if they got a mobile for PFB in Y6 they've all said 'No, I've only got it for DC2-4 because DC1 or DH have got a new phone'.
HTH