This is a perfect example of laws that will doing nothing to stop mass shootings but will cause problems for those that follow the law.
Most serious crimes are committed by those that have long records either within the school or out in society. How about enforcing the laws already on the books, right at the start of the criminals career. Give the criminal a reasonable sentence for the crime committed and have them serve the full sentence. If more than one crime is committed before they are caught, charge them with all and have the sentences be served one after another. Let's put them away and keep them there.
Expense, yes. Repeat offenders, no.