Answer: it's all to do with innocent until proven guilty laws.
Basically, in the UK, you can't push a view in the media that someone committed the crime until they are found guilty. You can say blah happened and it is suspected that blah happened but if you say they did it before they are ruled guilty you open yourself up to legal issues.
This continues over to use of photos etc where if you use photos that make them look "bad" then that entity may be accused of pushing the view before the suspect is found guilty.
But, once found guilty, you can do it. If you go look at the BBC News site today you'll find that they don't really use this image and now use mugshots and other images where she doesn't look happy/smiling.
As for why this specific picture is still sometimes used - it's really just to do with what users will recognise, because this case has been going on for years now and that's the picture they always showed. Sky News (the site in the link) does also have other non-happy pictures of her now.
When we say school bus it's an "everyone has a seat and is sat down" scenario.
With that established, I'm going to tell you that pretty much every EU country that runs school buses of that scenario does have seatbelts. We definitely do in the UK.
It clearly is possible. I'm not saying that the busses aren't otherwise built for safety in every way like many other commenters are saying, and maybe they wouldn't even provide extra benefit (which I doubt, but I see some commenters are citing studies), but it is definitely possible to add and realistic for kids to use seatbelts.