Maybe it will help you to memorise it if you think of these states as being similar to jobs. Jobs aren't necessarily permanent but people use "ser" to describe their occupations. Even a student (definitely not a long term thing) would say "Eu sou estudante".
I know that phrasal future tense (ir+infinitive, "eu vou ser") is commonly used in Brazil. Other than knowing that this exercise is practicing normal future tense ("eu serei"), how one know when one MUST use normal future tense rather than phrasal future tense?