[SOLVED] Modern fantasy with witchcraft for teens. [s]

[Goodreeds]

I read this book as a teenager, it must have been some time between 2002 and 2007. I must have been in the age group of the intended audience, I vaguely remember the book being in the 12+ or 14+ book section of the library I frequented at that time.

I read a German translation of this book, and I can give a description of the cover of the edition I read: It was all in black and had a portrait photo of a teenager filling most of the front cover. She had a lot of dark, wavy hair, filling up all the space on the cover around her face, and really pale skin.

And finally, what I remember from the plot: The main character, Kate, is about 15 years old and a witch. She lives with her mother (or aunt, or someone closely related to her), who is also a witch, and taught Kate about witchcraft. The mother (or aunt) runs a shop for magical trinkets and curiosities. I believe this shop was located not in the small town the story takes place, but on a small hill near the town. The building where the shop was housed, was also the place of residence of Kate and her mother.
Kate attended school in the nearby town. The main part of the plot starts when a new kid joins her school, his name is Thomas.
Rather quickly, Kate senses that Thomas is a powerful wizard, but he does not know he is. Kate and Thomas become friends, and Kate introduces him to the concepts of witchcraft. Thomas is reluctant to enter this world of magic, however, and Kate and he have a fight and avoid each other after that. Kate is desperate that Thomas has to come to term with his abilities, as he might pose a danger to others if he does not.
When Thomas accidentally causes a tornado (or, it might also have been an earthquake -- some natural disaster anyway), that brings down part of the school buildings, he finally gives in and eventually starts taking Kate seriously about witchcraft.
I don't remember if this is the complete plot of the book, unfortunately.
I also don't remember whether or not Kate and Thomas get romantically involved with each other, but I think there was a sequel to this book where they do.

Note that I'm not sure if the book actually talked about "witchcraft" or "witches", but these are the words I use to describe what I remember. Also note that I only read this book once quite a while back, so I might remember one or the other plot detail incorrectly.

I don't remember any other helpful details such as publisher or author or even original language. Though, I'm quite sure it was English, given the names of the main characters, and the setting.

I tried searching the Internet with plot details to find this book, but Kate and Thomas are really common names, so I haven't spotted anything that could be it yet...

[Andria]

It kind of sounds like Old Magic. The girl is Kate, but the boy is Jarrod Thornton, which maybe you misremembered as Thomas or it was changed in translation?

[Mir]

Maybe something by L.J. Smith?

[MJ]

It sounds a lot like Old Magic!

[Courtney]

It does sound a lot like Old Magic but there are some parts that she mentions that don't fit, like the sequel. There is no sequel.

It's funny because someone else was looking for this book just a few days ago, too!

[Goodreeds]

Yes, you guys were right. It is indeed Old Magic.

What I was thinking of as the sequel, was actually just Part II in the book, so that mystery has been solved too.

Thank you very much!