The fact that they had to not only do the work, but meet with the advisor on this, and didn't, seems to indicate more issues than just forging the signature.
Are you sure that maybe this isn't work that was done a few years ago by other students who you may not have had? Perhaps these kids knew the advisor would spot it right away. I'd meet with the advisor with the work for starts, then go from there. But there is something more than just no signature. How hard can it be to meet with an advisor to have him or her sign off on something? This is more than them just forgetting to staple the papers or put them in a binder.
And, again the issue of forgery. How would you feel if these kids went to the advisor, with a forged note from you, letting them go there during another class?
And at this point I think an "F" is fair, with a letter to the parents explaing why. These kids knew what they were up to. Let them take the consequences.
Are you sure that maybe this isn't work that was done a few years ago by other students who you may not have had? Perhaps these kids knew the advisor would spot it right away. I'd meet with the advisor with the work for starts, then go from there. But there is something more than just no signature. How hard can it be to meet with an advisor to have him or her sign off on something? This is more than them just forgetting to staple the papers or put them in a binder.
And, again the issue of forgery. How would you feel if these kids went to the advisor, with a forged note from you, letting them go there during another class?
And at this point I think an "F" is fair, with a letter to the parents explaing why. These kids knew what they were up to. Let them take the consequences.