Don't scratch, it will make it itch more as well as damage your skin.
If your legs itch, put on a cool dampened cloth or have a cool shower.
If you still have the problem with the itching, ask a parent or guardian to book an appointment for you to see your doctor.
Itching can be caused by allergies and that can be from absolutely anything. It could have been an allergy to the material of your trousers, or from an allergy to the detergent that they were washed in.
It can also be from some bugs. Scabies burrows under the skin and that can cause extreme itching, it is also contagious.
Rather than olive oil, try and get some virgin coconut oil. This is antifungal and antibacterial as well as moisturising. It can be eaten straight off the spoon as well as used in cooking.
From the pharmacy, you can purchase calomine lotion or calomine based cream to stop the itch. Applying natural set yogurt will also cool down the skin and help to ease off itching.
If they are caused by bug bites or by scabies, you would need to treat the bug bites with an antiseptic cream and scabies needs to be treated by a special lotion which can be purchased from the pharmacy or prescribed by the doctor.
As you are young, the skin will heal and the dark spots will fade in time. If they are scabs, leave them until they drop off on their own. If you scratch them off, you will end up having scars.