Well, after suspecting the other teacher (her mentor), the security guard, the Latino bad boy, the Good Boy who brought the case to the attention of the Cold Case crew.
1. Latino boy was caught with drugs in his back pack - this was observed by Darnell who was staying after to help the teacher.
2. He insisted that they weren't his and further suggested that Darnell's sick crush on the teacher led to her murder.
3. The questioned Darnell.
4. They discovered (after whathisname - the big guy bribed the clerk with dinner and a show - that the gun was one that had belonged to the father of a student and had been confiscated by the security guard.
5. The security guard admitted that he had leant the gun to the mentor teacher.
6. Mentor teacher confessed that she requested the gun because she'd been beaten up by some former students and no longer felt safe. And that her purse had been stolen from the teacher's lounge.
7. Raymondo was accused of stealing the purse. But he didn't.
8. Because guess who had a cocaine habit and left the school rather than being fired?
9. That's right. Teacher 3. Laura confronted him and gave him an ultimatum. He pulled the gun on her and she ran away. Then he shot her right where Raymondo could see him.
10. Ends up with Darnell doing the newspaper project with his class, Security Guard still drinking in his car, and Raymondo studying for his GED.