I learnt from my high school English teachers and grammar books that the definite article “the” is indispensable before any ordinal number. But I’ve seen sentences like “I am in eighth grade” and “World History is sixth period”, and native speakers told me that’s natural and common. Could you please explain a little for me?

By the way, how do you read “Dec. 17, 2010” — “December seventeenth” or “December the seventeenth”?

Thank you!
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Every language is divided into two major forms of usage: spoken language and written language. Some of the things allowed for the spoken language are never to be used for the written language. Another thing to remember is that there is the ROYAL ENGLISH and many other variations of the English language. What variation of the English do your NATIVE SPEAKERS use?

Yes it is natural and common particularly in America but your English teacher is correct there should be a "the". I will leave it to a native American to say if this is now considered correct in every day speech.
DearYolandahow do you read “Dec. 17, 2010” — “December seventeenth” or “December the seventeenth”

December the seventeenth (or even) The seventeenth of December. The British tend to put the date before the month.
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Thank you, inbozz!
Thank you, Louise!
inbozzthat there is the ROYAL ENGLISH
I have not heard this term inbozz. Do you mean "The Queen's / King's English"?
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It is just another way to say British English
Can you tell me please where you have heard or seen that expression because I have never heard or seen it. Do you have a reference for it? Is it used in the USA or maybe Australia? Sorry but I have not found any references for it. It would be useful for me to have a reference in case anyone asks me about this in the future.

Thank you

Search the web for The Royal English Dictionary. The term is rather old and I believe I've heard it from the team of professors from The London School of Economics I was privileged to translate.
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