This is what I came up with as an introduction to my essay titled "dress codes in schools". could you please restructure it or tell me how to make it flow? Thanks

Tuesday, April 20, 1999 the state of Colorado woke up to one of the largest, most violent mass-slaying of students in school in our country’s history. Two 18 year old boys, Eric Harris and Dylan Klebold started shooting and spreading bombs all over the school. Their senseless act of violence left 15 dead: 12 students, one teacher, the two perpetrators committed suicide and many more were injured.
Investigations revealed that the two boys belonged to a gang and were picked on by other students because of their apparel and grooming. This tragedy may have been avoided if the school had a dress code. While there may be opposition from students and parents, dress codes should be enforced in schools as they encourage discipline among students.

Is this supposed to be one paragraph? Why the separation after "injured"?

You need to pay attention to comma uses:
http://grammar.ccc.commnet.edu/grammar/commas.htm
See especially #4 and #10

... and to uses of the hyphen:
http://grammar.ccc.commnet.edu/grammar/marks/hyphen.htm
See c. about half-way down.

... and to fixing runons:
http://grammar.ccc.commnet.edu/grammar/runons.htm

... and to verb tense sequencing:
http://grammar.ccc.commnet.edu/grammar/sequence.htm

The content is really good!!

Would it better as one paragraph? Thank you.

Restructured version:

On Tuesday, April 20, 1999, Colorado faced one of the most devastating acts of violence in our nation's history—a mass shooting in a school. This tragedy occurred when two 18-year-old boys, Eric Harris and Dylan Klebold, unleashed violence by shooting and planting bombs throughout the school. Their ruthless act claimed the lives of 15 individuals, including 12 students and one teacher. Additionally, the two perpetrators tragically took their own lives, leaving many others injured.

Further investigations into the incident revealed that Harris and Klebold were part of a gang and were subjected to bullying by fellow students due to their clothing and appearance. It is conceivable that this tragedy could have been averted if the school had implemented a dress code. Although implementing dress codes may encounter opposition from students and parents, they should still be enforced as a means to instill discipline among students.