We usually don't use"孩子们拉着手走出校门去", because "走出“contents the meaning of"去”。 In this sentence, "孩子们拉着手走出校门去了","去“is also unnecessary, and "了” means having finishing something, like perfect tense

every character has a distinct meaning of its own. 孩子： refer to child in general 们：s，ren 拉：pull/ take a hand : http://image.baidu.com/search/detail?ct=503316480&z=0&ipn=d&word=牵起&step_word=&hs=0&pn=4&spn=0&di=22425505190&pi=0&rn=1&tn=baiduimagedetail&is=0%2C0&istype=2&ie=utf-8&oe=utf-8&in=&cl=2&lm=-1&st=-1&cs=3750106273%2C3167938795&os=2403636748%2C1367765406&simid=0%2C0&adpicid=0&lpn=0&ln=1974&fr=&fmq=1486967768546_R&fm=index&ic=0&s=undefined&se=&sme=&tab=0&width=&height=&face=undefined&ist=&jit=&cg=&bdtype=0&oriquery=&objurl=http%3A%2F%2Fn1.itc.cn%2Fimg8%2Fwb%2Fsmccloud%2Frecom%2F2015%2F07%2F23%2F143763794862475695.JPEG&fromurl=ippr_z2C%24qAzdH3FAzdH3F4p_z%26e3Bf5i7_z%26e3Bv54AzdH3Fda8ca0dnAzdH3Fg980nbnclm_z%26e3Bfip4s&gsm=0&rpstart=0&rpnum=0 拉着：pulling 手：hand。 you need a object pulled 走：walk，go，take steps towards a direction 出：out from sth。 teacher will say ：”get out“ to mean out from the classroom. chinese teacher will say：出去！ or 滚出去！ 去：it's a state. you speak loudly：qu（the fouth tone）。 sound like someone away from, gone, was, disappear gradually from your sight. such as 远去 校门：the school gate so, do you know what's difference between them?

All three are correct. 孩子们拉着手走出校门。 ---------- You are telling about a scene, in a progressive aspect . As you know, there is in this description no time/tense information, which will be revealed by the specific context. 孩子们拉着手走出校门去。------ practicaly all the same as the above one, only adding a little more meaning of the direction/tendency of the motion. 孩子们拉着手走出校门去了。---------- by 了，you're giving a meaning of the past time.

孩子們拉着手走出校門去(X 孩子們拉着手走出校門去了(X 孩子們拉着手走出校門了(O

孩子们拉着手走出校门去。This sentence is incorrect. No "去“ at the end of the sentence.