Let me put grad school in this perspective:
In HS, you go until 3:30 or so and then you get the rest of day off. Mommy and daddy take care of most of life's pressing needs, and you can have a lot of fun.
In college, you go to class whenever you have your classes scheduled for. You need to take some of your personal time aside for projects and work because even though you have fewer classes, the rigor of your assignments is an order of magnitude more. Mommy and daddy might take care of some pressing financial needs, but you have to wake your ass up, set your own bed time, make your own meals OR pay someone else to do so (dining hall, restaurants), you start learning some personal finance, and you have fun when you can.
In grad school, your first year is kind of like college, except by now your parents have told you that you're on your own, you'll likely have what equates to a job in an assistantship. You can have a little fun until your first summer. Then during the summer, you are expected to put your nose to the grindstone in terms of your research.
Then it goes downhill from there...you'll probably still be teaching, but your advisor (I call mine my boss) still expects 50+ hours of work from you. If you get a research assistantship 70+ hours will be expected from you. So, while teaching, doing research, and maybe finishing up an odd class or two you want/need, you have to start preparing for whatever weeding out process your school uses to separate the wheat from the chaff. Most schools will use some combination of an oral examination in front of a committee and written work. Oral examinations are the academia equivalent to being waterboarded, electrocuted, and stretched on the rack simultaniously. Its the most humiliating experience - even when you pass with flying colors you still feel like a dumb piece of shit and you realize how FREAKING smart the professors really are. It's like they turn on a switch and become omnipotent. And there are three to five of them, so they cover each others asses and try to blow you out of the water. If you have free time...you probably aren't cutting it. Third year...another round of weeding. So you have to prepare for that, along with doing even more research because at this point you're more competant and knowledgable, and your research will be even harder because by this point you'll be working on your own project instead of wrapping up the work of the previous grad student or post-doc.
Survive year three? More research, more hoops to jump through and red tape to go through before you graduate, papers to write, conferences to attend and present at, tack on some more responsibilities because you're now the senior member of the group...and did I mention more research? Oh, and you're probably still teaching or doing something to pull down a paycheck to keep a roof over your head and food on the table. Damn, all this time you're also going to have lots of meetings...meetings for the class you teach, group meetings with your research group, one on one meetings with your advisor, departmental seminars both internal and external, and probably the occasional training meeting. And all these meetings are from day one so get used to it. I cram almost all of mine in Fridays since that's when external departmental seminars are. So my Fridays are spent doing mostly administrative stuff...no time for research, guess I'm working Saturdays and Sundays.
Hell, I haven't even began talking about writing your dissertation. I'm working on mine right now. First chapter is usually a comprehensive review of what was previously done and also a justification for your research. Mine is 22 pages, with 257 references. That's 257 papers/reviews/book chapters you have to be INTIMATELY familiar with. After that, you have to write up all your own work. I'm not even half way done on that and I'm already over 160 pages. I'm going to end up writing a 300+ page book that NO ONE IS GOING TO READ, EVER.
I had no clue that THAT is what grad school really is when I started. And there you have it.