I saw this movie as a child and really enjoyed it. Anyone who has had an unrequited crush on someone completely unreachable, ie teacher, much older adult, will find this movie particularly moving. As will many high school students, who would love to have been given the prospect of running their school for a week - changing uniforms, abolishing homework, introducing half days etc. Danny is hopelessly in love with one of his teachers, but knows he could never have her, not just because of the age gap and the fact that he is her pupil, but because she is dating the high school coach. Magically, he becomes an adult (but still with the brain of a 14 year old) and sets about wooing her. It is a non-demanding, sweet, enjoyable little fantasy which I have never forgotten, even though I have only seen it a couple of times.
Just don't set about trying to analyse the plot and just sit back and suspend your disbelief for a while.