What level should a shorter-duration “Mass Protection from Energy” be?

The group I DM for (7 characters – Bard, Cleric, Druid, Fighter, Ranger, Rogue, Wizard – all level 7) will soon go up against an Adult White Dragon. (They don’t know yet. Heh.)

I would like to gently introduce them to the art of researching enemies and preparing for encounters, with the help of their conveniently-high-level spellcaster mentor (class left open for flexibility). In the present case, he would suggest Protection from Energy against cold damage.

Unfortunately, this 3rd-level abjuration only targets one creature – and it requires concentration. So our three characters capable of casting the spell (the Cleric, Druid and Wizard) could, between them, only protect three characters. Conversely, the duration of one hour is really far too much if they can choose the time and place of the battle.

So I am thinking of their mentor handing them (or their finding) a scroll of Mass Protection from Energy, which functions just like Protection from Energy, but protects up to ten creatures, with a duration of up to one minute (with concentration).

Question: what would be an appropriate spell level for Mass Protection from Energy?

Is there a mass version of Protection from Energy? is related, but for dnd-3.5.