function in aggregate applied to specific columns only

I want to use aggregate with the mean function on specific columns

gender <- factor(c("m", "m", "f", "f", "m"))
student <- c(0001, 0002, 0003, 0003, 0001)
score <- c(50, 60, 70, 65, 60)
basicSub <- data.frame(student, gender, score)
basicSubMean <- aggregate(basicSub, by=list(basicSub$student),  
FUN=mean, na.rm=TRUE)

This doesn't work, one cannot take the mean of a factor (gender).   
Is there any way of specifying which columns to use for the mean?  I  
want to aggregate by student, obtaining mean scores, and assume any  
other factors are unchanging in a specific student, ie. gender.

Thanks
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