I work in a government institution that gives an additional three dinars per hour I work. The manager combines students into a single session because attendance is low, and the material taught to all groups is the same. I am paid for two hours, although I actually taught only one session, noting that my transportation costs one and a half dinars. What is the ruling?

The only amount you are permitted to take/have is for one hour work. And Allah Knows Best.

Do the Jinn have anything to do with the disappearance of objects?

Jinn are one of Allah creatures, and whosoever lost anything should look for it and never accuse niether the Jinn nor humans of stealing it. And Allah Knows Best.

Is wearing a cap permissible for boys, or girls?

All Perfect Praise is due to Allah, The Lord of The Worlds.

In principle, anything is permissible sol long as there is no Sharia evidence prohibiting it, thus wearing the cap by boys is permissible as long as it isn`t done in imitation of the disbelievers. Moreover, a Muslim woman should avoid all that which attracts the attention of men because whatever leads to the forbidden is forbidden. And Allah Knows Best.

What is the ruling on performing ablution (wudu') and ritual bathing (ghusl) with Zamzam water?

It is permissible to perform ablution and ritual bathing with Zamzam water. However, scholars disliked using it for cleaning after relieving oneself (istinja'). And Allah the Almighty knows best.