Do NOT do a science "because you get good grades in it" at GCSE. A-Level is a whole different story. Only take a science subject (and in fact, any subject) if you sincerely enjoy it. Chemistry at A-level is a whole lot harder than the simple things you're doing now at GCSE.
Psychology isn't a waste of time. No A-Level is a waste of time if you have the interest in it, and those which say they are more often than not the holier-than-thou types. People think Psych's a soft subject, but it isn't. It may not be "traditional," but that doesn't make it soft or easy. And anyway, if you're going into teaching the only subjects you might "need" to take are whatever subjects you are intending to teach.
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