Wiktionary
write out
vb. (context transitive English) To write in full length or expanded form.
WordNet
Wikipedia
Write out
A write out is a punishment used in schools in which a misbehaving student must write something out. It is different from writing lines as, instead of writing something out a number of times, the student might write out a page, chapter or paragraph of their schoolbook, workbook or novel. In the case of non-written homework ( spellings), the student might have to write the spellings a number of times.