You need a lawyer for a current, legal and correct answer.
Just as much as if you use a real gun.
If its in California States
5-10 years.
Depends on what the charge is.
0 to the Death Penalty depending on circumstances
0 to Life Without Parole depending on the circumstances
It depends on what state the school is located in.
Depends on the charges and the place where tried. No one answer.
What gun?
State time: Up to 15 years. Federal time: Up to 15 years.
You need to consult with a lawyer familiar with firearms law.
Unless you have a higher priced or accurate gun, buy high end bb's and take the time to sight it in properly, a red dot or any optics does not do much for the gun.