The F-1 visa is a non-immigrant visa, but it does not mean that a F-1 visa holder is not allowed to apply for the Green Card. A non-immigrant visa holder, such as F-1 and J-1, may change its intent thereafter receiving a nonimmigrant visa.

It is not true that a non-immigrant visa holder must change its visa type to a nonimmigrant visa such as H or L visa, which allows dual intent, before a  Green Card petition can be filed. Generally speaking, anyone can file an immigration petition at any time if he/she meets the qualification requirements. But, if your current F-1 visa is not long enough for you to finish your degree, it is better to wait the Green Card application until you have extended your F-1 visa.