When Evil Calls - C E Marshall - Bog

A whole class of boys disappear without trace from their school. Another set of boys apparently witness what happened but are physically prevented from describing what they saw for some months.
This latter group of boys separate to follow their own careers, marry and raise children, but are eventually all brought together again in an engineering capacity - despite earlier interests. Working together they devise new alloys and electronic communication devices. They build a model of machine they believe will travel through space - at the same time secretly building the parts for a full size machine.
The model is tested in front of witnesses and performs perfectly but disappears from all monitoring systems for most of the test flight.
While testing the new communication device a message is received from their old school friends who vanished those years ago. It takes some convincing of others that these boys (men now|) are still alive but permission is given for the full size machine to be built.
Whole families are smuggled aboard the machine which, rather than going for a test flight, heads for the planet where their old friends are waiting.
On arrival they find that all the people there have to be underground at night and in well lit places; any person, or animal, left outside at night is found dead the next day.
They explore the planet, make friends with unknown animals, but are caught outside at night. A call for help is answered and they all survive, but now have a mission to destroy the darkness.
They return to Earth to pick up people versed in ancient magics and return to Paxlene where, after adventures the darkness is defeated.
An attempt is made to return the special people to Earth but fails as the Earth is about to be hit by a giant Asteroid. They are able to cocoon parts of the Earth and shield them from the catastrophe, but it all happens too quickly for anyone to be returned home.
They try return to Paxlene, now free of evil, where they plan to help to build new societies out in the open, but something takes over their controls.
The story continues with 'Wormhole' , and will conclude with a third tale which has a working title of "Restitution".