Why is this estar ? when if I'm on vacation "Sou turista". I suppose, it's "Eu sou aluno, e estou na oitava série" ?
Yes. 'Eu sou oitava série' or 'Eu sou 8ª série' work as well. '8ª' is an abbreviated form to write the ordinal number 'oitava' (feminine).
Notice that if you use 'ser', you do not use the preposition 'na' as in 'na 8ª série'. So,
'Eu estou na oitava série' = 'Eu sou oitava série' = I'm in eighth grade.
Not sure about that, because it depends entirely were you start counting, 8th grade is the nineth year of education if you count Kindergarten (Nursery School ?) as a year of education.
The American grades are like this: Preschool - usually 4 year olds, though some start as young as 2 1/2, and some stay in pre-school after they turn 5 - Preschool is optional Kindergarten - must be 5 years old by the cut-off date at the beginning of the school year 1st grade starts after the kindergarten year (6 year olds) Most students complete 12th grade and graduate from high school when they are 18 College comes after high school/12th grade
The divisions in K-12 are confusing. Typically, the grades are divided into Elementary - K through 5th or 6th grade Middle school - 6th -8th or junior high 7th - 9th High school - 9th through 12th.
That was 2nd or 3rd year, it's so far back, when I was of that age, in the UK.