Factors that Influence Second Language Acquisition

Second Language is the term used to describe the acquisition of any language after the mother tongue. There exists a lot of controversy about the order in which a Second language is acquired, because this is not clear, I would mainly point at the Sequence of Acquisition.
In second language acquisition the term Target Language is used to refer to the new language being learned. The Researchers have found that there are internal factors that do not let everyone learn a language in the same way. This is what they call free variation. This may indicate different learning styles and communication strategies.
Free variation may in itself mean that the language has not been freely acquired. In other words, variation is mainly found in begining learners. It seems that beginning learners are still trying to figure out what rules govern the use of alternate forms. Researchers found that many factors influence the target language acquisition. One of these factors has been identified as psychological, and it is the planning time. The more time the learner has to plan and produce the more regular and complex his production is likely to be. This means that the learner may produce better target language forms when they have more time to plan, then what they usually have in a conversation.
Another factor according to researchers is the Affective Factor. For example, learners in a stressful situation may do poorly in their target language, but they may do well in a comfortable setting. In other words learners may do poorly in a test and good in a relaxed environment.
One of the External factors might be the attitude of the surrounding society. If the learner of the target language percieves that the society approves of his learning the target language. Then, the learner may learn the target language with ease. The opposite will happen if the surrounding society disapproves of the learning target language.
Another External factor is the Community Attitudes towards the language being learned. For example, the Navajo Children when they are learning English, they experience the disapproval of all of the Navajo's community. This in return makes it almost impossible for the Navajo children to learn English, because they are experiencing the disapproval of all of the Navajo community. The opposite is true when the community attitudes approved of the target language being learned.
Another External factor that we do not need to forget is the group dynamics in the language classroom. Whatever the group (classroom) accepts or does will determine the individual's learning of the target language.
According to Nation (2001) in his review of various studies, he found out that only if 98% of the words are previously known by the language learner; then the extensive reading will be effective. If not it will not be effective.
In other words, my point is should we have these SLA in the regular classroom when they are not capable to understand 98% of the material that is being taught to them?
http://en.wikipedia.org/wiki/Second_language_acquisition