ANSWER: Writing activities must take place in class. And, when we say writing, we mean productive writing activities which are specifically designed so learners can produce in writing the knowledge they have just acquired or they have acquired so far.
For example, the following is an in-class writing activity for Elementary learners (A-Senior, B-Senior), who have just been taught "There is/There are" and who have as previous knowledge: the verb to Be, plurals, numbers, and a considerable number of vocabulary items which are usually taught at these Levels:
1. T. gives to individual learners different numbers from 1 - 10 (plastic numbers or numbers on Flash Cards (F/Cs). T. asks learners to think of any items according to the numbers on their F/Cs. S/he
can specify the categories, e.g., clothing, fruit, food, household objects,
etc., according to the vocabulary items the learners have acquired so
far. T. gives 1- 2 examples: "three apples", "six desks", etc.
2. S/he calls upon individual learners to say the items they have thought
of, e.g., five peaches, two cats, one sofa, etc. Each learner has been
given at least 5 - 6 different numbers from 1 - 10.
3. Then, T. asks learners to write 5 - 6 sentences, using the items they
have just called out. T. gives 1 - 2 examples, e.g.,
"There is one can of orange juice in the fridge."
"There are five roses in the vase."
4. T. sets a time limit. When time is up, T. asks learners to proofread their
sentences and to correct any errors. Then, s/he asks learners to
Report Back by reading some of the sentences they have written.
5. At the end, T. collects the papers in order to correct them later.
In class writing takes place mainly because this is the best way for the T. to train her learners to produce writing in a well managed manner:
a. Clear instructions which need to be written out, especially when
learners are asked to produce paragraphs, letters/e-mails or
compositions.
b. Careful reading of the instructions and making sure learners know
exactly what they are to do.
c. Brainstorming and generating ideas.
d. Creating an outline.
e. Composing (writing the task)
f. Proofreading and correcting possible errors caused by
absentmindedness, haste or stress, and
g. Concluding the writing activity within the time limit.
No learner can discipline himself/herself to manage his/her writing in the way described above when s/he is at home. On the other hand, teachers cannot have personal knowledge of each learner's ability to follow this procedure which contributes greatly to his/her progress and development into a skilled writer.
Any writing assigned to be conducted at home is Homework or, in the case of Exam Preparation classes, extra practice.