Class  Students Gender Height Year_1999  Year_2000 Year_2001 Year_2002
     1      Mark     M     180      80        54         22       12
     2      John     M     234      0         59         32       62
     1      Tom      M     124      0         53         26       12
     2      Jane     F     180      80        54         22       0
     3      Kim      F     140      0         2           3       32

Class  Year_1999   Year_2000   Year_2001  Year_2002
     1       1            2            2         2
     2       1            2            2         1
     3       0            1            1         1

Number_obs = df %>%
    group_by(class) %>%
    summarise(count=n())