What does 背 mean in this sentence?
I saw that "He's the tallest in class." is translated as "彼はクラスで一番背が高い。", but I just can't understand why 背 is used...

In another sentence it is said that "This is the tallest building in this town." is translated as "これがこの町で一番高いビルです。", so I thought the sentence above would be translated as "彼はクラスで一番高いです。", but it wasn't...

Can anyone help me pleeeeease??? '-'

Question

What does 背 mean in this sentence?
I saw that "He's the tallest in class." is translated as "彼はクラスで一番背が高い。", but I just can't understand why 背 is used...

In another sentence it is said that "This is the tallest building in this town." is translated as "これがこの町で一番高いビルです。", so I thought the sentence above would be translated as "彼はクラスで一番高いです。", but it wasn't...

Can anyone help me pleeeeease??? '-'

Answer

Of course the adjective 高い means tall/high by itself, but for describing the height of a person you need to use the expression you mentioned: 背が高い. 
背（せ） means 'back' (part of human's body). So instead of saying "he is tall" they say "his back is tall" in Japan. Saying 「彼はクラスで一番高いです」 would probably sound as odd as saying "he's the highest person in the class" in English.

Answer

When Japanese measure a height of body, they put the back on a pillar and write the mark on the pillar. Then, they measure the height of the mark. Precisely, the height of back is measured.

Answer

Ok, thaaaanks for your help. '-'

Answer

Hi, Karol's answer is perfect, please remember that "tall" for person is 背が高い. We use 高い for other things, too, like "tall / high / expensive".