Man this is a bummer. Speaking from someone who had a very close friend pass just months after HS Graduation back in 2004 in a car accident I hope to offer a glimpse into the mindset of your students.
Having other close people pass away since that accident (including my mother) I can honestly say that the feeling is the exact same. No matter the age, whether i'm 18 or 29, that deep sorrow is exactly the same. At least it was for me. The closer friends of the student will be very depressed, and for quite a while, possibly through the rest of the year, be very despondent. They will be agitated by the students who weren't that close and don't seem to care. Likewise they will be frustrated with people who may have picked on that student and are now showing feelings of sorrow because they can't yet understand the gravity of the situation and just because someone wasn't personally close with someone doesn't mean that this situation hasn't brought other memories up for other students (I hope that makes sense).
Treat these students 100% as if they are feeling the exact same thing as you. Because, although different, they are very similar. But the main difference is that this might be the first tragedy for many of them. They don't know how to self cope with situations like this. And it will be a moment that will stay with them and they will use it as a gauge for other tragedies in life. Our friends still get together on the day our friend passed away over 10 years ago.
my main peice of advice is this: Definitely Acknowledge it.
When my friend passed his two younger siblings were still in middle and high school. The school did nothing in rememberence for him (because he had been expelled and didn't technically graduate i guess). But being that this was a student, i'm sure the school will have counselors on hand and just keep an eye out for those students that are both quiet and don't actively seek help. Let them know that you are there for them. I'm a 6'2" 300 pound man, and I was this size in high school. I would have gladly cried on a teachers shoulder. In fact I was in my first year of community college when this happened and my english professor really made sure I was OK and even walked me to the counselor's office and urged me to talk to them.
I hope this offers some sort of advice for you. All the best and sorry to hear about this tragic loss.