over-optimistic calculation; counting one's chickens before they're hatched
Abbreviation of the proverb 取らぬ狸の皮算用. Used to describe making plans based on uncertain outcomes, often with a critical or cautionary tone.
See also: 捕らぬ狸の皮算用
Talking about a celebration for getting into school before even passing the exam is a classic case of counting your chickens before they're hatched.
His business plan was criticized as nothing more than an over-optimistic calculation.