PTO - Please Turn Over
"Please Turn Over" in the form of an acronym is mostly used to request the reader to turn the page after reading it. It is kind of request and a signal to the reader, that there is some more for you on the next page.
If the student is writing a Paper of Science. In a ninth page he left last 5 lines of the page and has drawn a diagram related to this answer on the next page, so to let the reader know about this portion of the answer, the student will write PTO in the blank lines.
PTO - Paid Time Off
Paid Time Off, also known as Personal Time Off is a policy in some organizations which allows employees to take days off with pay. Suppose an employee gets 14 days vacation, 2 personal days, 8 days sick leave and 10 days paid holiday, then under the PTO plan total of 30 days would be credited to Employees leave Bank.