Disagree. It can be written either way.
"turn sth in
1 to give back sth that you no longer need: You must turn in your pass when you leave the building.
2 (especially NAmE) to give sth to sb in authority: They turned in a petition with 80 000 signatures. I havent even turned in Mondays work yet.
3 to achieve a score, performance, profit, etc.: The champion turned in a superb performance to retain her title." http://www.oup.com/oald-bin/web_getald7index1a.pl
- Note that the phrase is found in the Oxford Advanced Learners' Dictionary under "turn sth in".
(also "turn sb. in")
" to write a profile of Guevara for the New Yorker magazine, but according to
his friend, Paul Lee, "he got drowned in it and never turned it in"." http://www.guardian.co.uk/cuba/story/0,11983,...
""I liked Napoleon and I went to The library and I Read six or seven Books and
turned it in. " http://www.guardian.co.uk/arts/features/story...
"This is the brother that turned the Unabomber in. " http://www.guardian.co.uk/g2/story/0,,1496347...
"I had never turned the piece in to my editors at the New York Times because I
felt I had not solved the mystery as to the quality of Reagan's intellect. ..." http://www.guardian.co.uk/uselections2004/com...