We already have that law, so the only thing up for debate is interpretation? Which legal experts are busy with debating now in public discourse in Swedish media, with no clear consensus except that it should be tried in court. I understand what you mean by slippery slope, but if everything is a slippery slope we would never be able to legislate anything. And let me remind you, both Sweden and the US have already imposed certain limits to the right to free speech. Defamation, for example, is not protected speech.
I disagree that a public school isn’t a public place, but you’re technically right. It doesn’t really matter in the eyes of the Swedish law though, arguably it would be worse legally if the student had carved the swastika on a public playground outside, rather then in a semi-public spot in a school.