I allow the other students to help, provided that they speak (the only students writing are at the board), speak only English (I take points off if I hear the native language), and that the teammates don �t go to the board.

Here �s an example: the subject is one man or boy, the verb is have, the tense is simple past. One student writes: "Mr. Sanchez, he had a nu car but was in accidente. Now car is no good." I count this as 17 points: 15 words in English (accidente doesn �t count because that �s Spanish, not English), correct verb, and correct tense. The student doesn �t get the point for correct subject, because in English, it isn �t correct to use  a name and pronoun together as a subject. I would give the point for "Mr. Sanchez had..." or "He had..." but not together. You might count off for "nu" instead of "new," but I don �t. The student also needs "an" in front of accident and "the" in "Now THE car is no good." I just point these mistakes out but don �t penalize. 

My students are adults, but they still enjoy this game a lot. Children and teenagers would probably enjoy it even more. Your classroom might get very noisy, so you might want to be sure your next-door teacher isn �t giving a test.

It �s also easy to adapt the game to practice other grammar you �re working on. I �ve also done prepositions or frequency adverbs instead of the verb tense. You might even tell them the sentence has to be a question or in the negative. 

Thanks for your patience. I hope this idea works for you.