It can be tough to judge a footballer's speed due to their varying styles. I was surprised to hear about Robben clocking great speeds because to me he always looked like his small strides weren't really getting him anywhere. Walker has a powerful, hamstringy, pro-sprinterish looking style, and the fact that he brought up the whole "who's fastest?" thing, and backed himself over 100m, I reckon he has his fair share of sports day badges.
Just demand full disclosure on their school 100m times so you don't have to worry about injuries.