What is actually considered "hacking"?

So I was looking around at websites one day and was realy bored… and my friend said something about "hacking" my schools website. So I got on there and and I think it literally took me about 10 seconds before I was in the admin panel to start defacing the site. I didn't have to actually bypass and passwords… it was pretty funny actually, the site is very insecure. www.fairfieldcityschools.com :D I just did a stupid little thing on the front page…. Pretty n00bish I guess, but I don't want to get into any trouble. My question is tho, since I didn't actually have to hack anything (PM if you want to know how I did it…. to prevent noobs from just going on there and doing whatever) is this considered ilegal?

Yes, it's illegal but the school probably won't care/take action unless you do something annoying like deface the site.

Do they know it was you?

What was change on the page ?

Note :

Web page of school are most of the time insecure. The web page of my school had a bug that let me edit all the teacher page (i didn't do it) but they fix it because some personn have abuse of that exploit. Also the password are never secure because it's easy to guess (the four first letter are taken from the name and after there is four random number).

Well I had images up there…. but they took them down, and since they did I guess I can tell my lame "hack" lol…. Really I just found the link that took me to the page where all the images are on, and i could upload my own images, name them as the same thing and overwrite the old images… That's all it was…. theres still one pic up if you click on schools > middle school lol… Lame and little I know…. but funny how easy it was.