Hi everyone

I'm currently a permanent resident(greencard holder) and I am considering studying abroad. Does anyone if there is some sort of student leave visa available for those wanting to study abroad but don't want to let there greencard become invalid(i was told that there is some sort of law that if you are a permanent resident and leave the country for more than 6 months your greencard becomes invalid)? I don't even know if it is a called a visa. If anyone knows any info on this please let me know.

thank you so much.