Speculators buy assets that they aren't going to use themselves purely to sell it to someone else. If speculators are selling to mostly to other speculators and transactions between speculators starts taking up the bulk of the active market and it starts driving prices up... that's a bubble. What happens is that at some point speculators will panic when other speculators stop buying the asset, and they race to cash out of the asset before everyone else does, except there's mostly other speculators who are also trying to dump the asset on the market...
Those Dutch tulips or 2000-era dot-com stocks are examples of this type of asset. These are assets that didn't have much value except to speculators. (Cf. you can live in or rent out a house, or collect dividends from other categories of stock, or melt down gold, etc.) As soon as there's a price drop... every speculator is terrified of being the last one holding the bag and sees every other speculator thinking the same thing and accurately see speculators as controlling the price and they all race like hell to dump the asset as quickly as possible. The price drops to near zero.
With an education it's difficult to imagine how this sort of thing could happen. In short, it's an exceedingly difficult asset to transfer. You can't sell your education. There doesn't seem to be a conceivable way that everyone would "sell" their education all at once. Also the process of buying an education takes many years to carry out and so doesn't seem so compatible with overnight frenzies. And the value of previously acquired educations doesn't drop if people buy fewer or cheaper ones; it might actually go up instead.
If people collectively decide Ivy league educations aren't worth buying anymore... it will probably be a trend that occurs over a matter of decades.
Just because something feels dumbfoundingly "expensive" doesn't mean that it's a "bubble".