I may be the teacher who initially gave you this advice. I agree with the commenters above. Although it's hard to give a complete justification in a short comment here, let me mention that there are significant differences between the pronunciations of modern RP (British) and General American: the vowel phonemes, rhoticity (whether orthographic "r" is pronounced) and the realization of /t/, among others. It's not realistic to expect to develop a native-like pronunciation if the model you are trying to imitate is always changing. In a classroom situation, you may not have much control over the accent you are exposed to, but there are lot of opportunities on the internet (italki, youtube) to hear your target accent. Language schools generally tout the advantages of being exposed to a variety of accents. Perhaps that's a little self-serving, given the international make-up of their staff, their coursebooks, the exam prep they offer, etc. It also gets them off the hook for teaching pronunciation adequately. Which variety should you target? That depends on your goals. Who is it that you *primarily* want to communicate with? Brits? Americans? Other native speakers? International (non-native) speakers? Both of the major standard accents have their challenges; I would not recommend choosing an accent based on a criterion of perceived difficulty. There is a legitimate argument that a native-like pronunciation should not the goal, but that's a topic for another day. Now if a student isn't interested in improving their pronunciation, then obviously none of this applies. Not everyone is interested in making the effort. In addition, as other commenters have mentioned, this advice doesn't necessarily apply to advanced learners.

I would definitely go with standard American English. It is what you will hear most and can be understood by all English speakers regardless of their nationality. The vast majority of Americans speak with the standard Midwest accent or pronunciation. It is what you hear on radio and television.

I think it is good advice as well. There are many people in America who speak English with a Mexican accent. If this is helpful to you, perhaps watch some English movies with Mexican-American actors. If you decide on the "American" accent, just make sure you also realize that there are several accents within the US. I would suggest sticking to American standard which is what you would hear on most national talk-shows and news. (Like the Today Show/Good Morning America and CNN or Fox News)

When starting out I think it´s generally better to focus on either American or British English. This will make comprehension easier for an elementary / pre-intermediate student when listening. As you advance to higher levels, in my opinion you should expose yourself to other accents (this will tend to happen naturally anyway). In terms of your own pronunciation, think about who will be listening to you. If they are mostly North American then it will be slightly easier for them to comprehend you if you have an American accent, but it´s not too important. Differences in accents are subtle when compared with incorrect word stress and intonation. If you have pronunciation problems then this is what you need to focus on along with phonetics. Both American or British English are fine for this even when mixed and using different teachers.