Is it acceptable or normal for professors to ask first semester grad students to teach a class?
Whether this is normal, uncommon, or outrageous will differ a lot from university to university. Where I did my PhD (in Austria), this would be standard operating procedure. Where I teach now (in Sweden), it would be uncommon but not really a huge deal as long as the teaching quality remained reasonable*. I am sure there are other universities where this would be unheard-of.


  Preparing and teaching a class isn't the problem, but I'm just wondering is this normal? I've heard of PhD-level students teaching classes in these situations, but freshmen like us? I'm honestly very concerned.


It would help to answer your question if you said what exactly you are concerned about? It sounds like you feel you can prepare the class well (and it also sounds like you don't mind doing so too much), in that case this sounds more like an administrative matter that should not concern you too much - ultimately, it's your professor's problem if your Dean of Studies (or whatever the comparable role in your university is) is unhappy with how exactly your professor decided to organize the teaching in your course.



*There would likely be some questions if the professor in fact delegated three classes in a row, but not because he delegated it to young grad students, more because designated teachers are in fact expected to do all, or at least the vast majority, of their teaching themselves, not just find somebody to do the teaching for them.