忙がしい or 忙しい?
I learned 忙しい in my school book, but The japanese script tool suggested I write 忙がしい. Is there a difference? If not, which one should I use?

Question

Answer

忙しい is the regular form, the が is being used here with something called okurigana. It is being used to disambiguate kanji as it has multiple readings. 忙しい can actually be read both いそがしい and せわしい however いそがしい is the common reading. I don't know much more then that but yes as Tomoko said, use 忙しい.

Further reading:
http://en.wikipedia.org/wiki/Okurigana#Disambiguation_of_kanji
http://tangorin.com/general/%E5%BF%99%E3%81%97%E3%81%84

Answer

忙しい　　correct
忙がしい　　not correct

You should use "忙しい".