Both terms mean "harassment". I tried to find out the differences by searching the Internet, but beyond this short Q&A in English:
いやがらせ means harassment.
いじめ is more serious harassment than いやがらせ. いじめ is done by group members, like classmates.
I just found resources in Japanese talking about it. I tried to go through those resources (see this and this) but I only could grasp that いやがらせ is for particular actions and いじめ seems to be more serious and a general situation of harassment rather than a single harassment act itself. Is this the difference in meaning between these words, or am I missing something else? Could you provide some example sentences that help to understand each word better?