Explanations

Joke 1

SIGN= A traffic sign.
The sign this boy is talking about is placed on the road / street to warn drivers to be careful and slow down, because children may be crossing.

Joke 2

This joke is about the use of "impersonal YOU" to refer not to the second person singular, but to people in general. The teacher uses YOU in an impersonal sense, the student takes YOU as the second person singular, so he thinks (or pretends) the teacher was referring to himself.

Notice here that direct questions (using a question mark: ?) have an inversion, but indirect questions (no question mark) don’t:
- How do you spell it? (do you spell)
- You ask me how I spell it. (I spell)

Joke 3

CHEMICAL= pronounced /kemɪkəl/.

The chemical formula for water is H2O. The number 2 is pronounced the same as the preposition TO, so instead of H-2-O, the student thought the teacher said H-to-O (= from the letter H to the letter O).

Joke 4

Obvious, right? Christopher Columbus discovered America, but at that moment, in class, little George had just pointed on the map where America is. Nobody else knew, so everybody considered that George had just discovered America (referring to the map). Here we can also see the usual confusion between "The United States of America" (the USA), the country and "America", the continent.

Joke 5

ME= This is the subject, so it "should" be "I", not "Me", but when we use the first person pronoun alone, we almost always say ME, not I. If we say "I" it would be correct, but pedantic, too formal.
If there is more than one subject, we usually use ME except when it is followed by a verb:

- Me and my brother went to the cinema yesterday.
- My brother and I went to the cinema yesterday.

Both sentences are correct, but the second one is considered to be nicer, more polite, because we are humble and put ourselves in a second place. To say "I and my brother went to the cinema" would also be correct, but pedantic.