If your F-2 visa is valid, you don't need to go back to consular's office in Nepal to apply for visa again. However, you need to have a valid I-20 with you to show at port of entry. Also, you need to get your I-20 signed by International student Advisor of your university before you leave the USA. That signature is like a re-entry permit. There is a designated place in I-20 where the advisors sign every time you go outside the USA.