Child of Child!

[nextgen]

if i have school class > then child table for class subjects > each subject may have be one or more teacher
i found theat frappe doesent support child of child tables or even multi select table or html
how to implemnt this

also can i extend frappe to add new filed type.

many thanks in advance

[nextgen]

may be the frappe team have the concerns about the child of child and condering that as bad design

but the question now why the table multi select not available for child tables?

many thanks.