Publish the proposed constitution well before the meeting along with a leaflet explaining how people can make amendments if they want to.
Further education institutions have a legal right for a students' union
If you are trying to set up a students’ union in a college of further education then you will most probably have a legally enshrined right to do so.
You can find out by requesting from the college a copy of the 'Instrument and Articles of Government'. This is a legal document that determines how the college is run; it is also a public document.
Check the Articles section for a clause
Check the 'Articles' section, for a clause about the students’ union, which should be worded in a similar fashion to the following.
"The students’ union shall conduct and manage its own affairs and funds in accordance with a constitution approved by the Corporation and shall present audited accounts annually to the Corporation.
No amendment to or recission of that constitution, in part or in whole, shall be valid unless or until approved by the Corporation".
This makes it very clear that the college has a duty to allow the establishment of a students’ union.
Sixth form schools
However, if you are at a sixth form school, then you fall under school regulations and will be totally dependent on the goodwill of the principal. In other colleges it will vary.