Daughters school just got put on lock down due to threat.

[Atrain1]

I do not have to much info at this time but my daughters school just got put on lock down do to some kind of threat. The only thing I know is they said it was some kind of prank and lifted the lockdown, but they will have police present for the remainder of the day. This is in California so I cannot do to much at this point but sit here and worry. What kind of threat would cause a lock down like this? Even if this is a prank it is a stupid one and not funny in any way. This is getting a little close to home for me as I am not comfortable with her in a public school to begin with.