It have two caveats.
First - Instead of "bet" you should say "I bet", it would be what one would expect, but then, it could be spoken English and all, I am the guy that never says never (no I am not Justin Bieber, please)! So, I will leave this at that.
I just would say "I bet" but it´s me, others may or may not say just ''bet".
Second - Again, in spoken it can be used double negatives. In written English, you can´t have two negatives at the same sentence, I mean it´s not advisable. Ultimately, the decision is yours.
makes a negative (many don´t know that, but hardly is negative in sense
, albeit it´s not directly "no"), and can´t
the other negative. So, it would be "can hardly" here."18th and finally school's out forever, I mean, for a while...(I) Bet college hardly can (or can´t) wait to have you there. (spoken)
"18th and finally school's out forever, I mean, for a while...(I) Bet college hardly can wait to have you there. (written)
I scoured at least two sites, BBC and the Guardian and I found just one hit of "I Can't Hardly Wait to..."
it was athttp://www.theguardian.com/film/2001/feb/08/news3
And even so, in an informal situation of sorts. What is a telling detail when it comes to written English.