This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Seats (fitted with backs) and desks should be provided, graduated according to children's ages, and placed at right angles to window wall. Eighteen inches per scholar at each desk and seat in determining lengths of same is sufficient with gangways 18 inches wide between groups and walls. In case of dual desks, the usual length is 3 feet 4 inches with gangways 1 foot 4 inches. In an ordinary classroom five rows of long desks or six of dual are best. A room providing for more than 60 children should not have more than four rows of long or five of dual desks. Long desks should be arranged that a teacher can pass between the rows. With dual desks this is unnecessary, as gangways give sufficient access. A teacher should be able to pass behind back row. Desks should be very slightly inclined. An angle of 15 degrees is sufficient. A flat desk has a tendency to make children stoop. A raised ledge in front interferes with the arm in writing, and the edge when used for writing should be vertically over the edge of seat.