Transforms wide enrollment data to long format with subgroup column. Also handles data that is already in a semi-tidy format (e.g., CTData.org data which has n_students and grade_level columns already).
Examples
if (FALSE) { # \dontrun{
wide_data <- fetch_enr(2024, tidy = FALSE)
tidy_data <- tidy_enr(wide_data)
} # }