At what time(s) of the day have you been visiting Cheam Station? According to the Office of the Rail Regulator, the station sees more than a million passenger "entries and exit" a year! http://orr.gov.uk/statistics/published-stats/station-usage-estimates
The majority of the passenger "events" involve the use of a season ticket; these people are most likely to be using the station at "commuter" times, so I would assume that the station is fairly busy when schoolchildren are arriving for school, if less so at school finish time as this is a bit earlier than the evening rush.
In comparison, our local station sees about 1.5m "entries and exits" p.a., again a majority of them season ticket use - it's pretty busy "up" in the early morning rush hour then again around 4pm with schoolchildren going home from one set of schools and arriving back from the ones further down the line; "down" there is a fairly busy period 7.30 to 8.30ish in the morning with local commuters / students and incoming and departing schoolchildren then the normal evening rush. In between times, though, there will be trains with fewer than half a dozen passengers getting on or off them.
Presumably when you chose the school, your DD's journey was one of the things you took into consideration?