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Should I start using STL from day one ...?

Yes. Using native arrays in C++ is error prone and simply bad practice 99% of the time.

For I/O, should I stick to cout, etc... ?

Yes.

Are there any online resources for C++ that are suitable to use for such young learners?

They aren't so young. Learning to program does not require wisdom, just motivation and an open mind. I can't think of anything taught in the last two years of high school that would better prepare a student to learn to program. Have you looked at Bruce Eckel's Thinking in C++? It's available for free download and very well reviewed. Please avoid the popular but horrible C++: How to Program and all the Sam's Teach Yourself C++ ... books.