Kowloon Junior School currently has two sites, this one next to Argyle Street, and another one over at Begonia Rd in Yau Yat Chuen. The latter is due to close down so the school can operate from a single site (i.e. this one) from 2011.
Here is Booth's description again: "surrounded by a six-foot chain-link fence on the other side of which was a steep drop to the dusty football field of another school".
Assuming the school hasn't changed location since the fifties (I know, BIG assumption) I wonder if the adjacent football pitch (which belongs to King George V School - the 'other' school perhaps?) is the same one Booth saw. Judging from my road map it is indeed on the lower side of a steep slope that drops down from the KJS grounds.